Water-holding carrier for plants

ABSTRACT

A water-retaining support for a plant exhibiting a water-retaining ability comparable to that of a polyacrylic acid-type hydrogel without inhibition of root origination or root elongation. The water-retaining support for a plant includes a hydrogel-forming polymer having a calcium ion absorption of less than 50 mg per 1 g of its dry weight and having a water absorption magnification in ion-exchange water (at 25° C.) of 100 or more. When the water-retaining support is used, a plant may be supplied with sufficient water without suffering from a deficiency of calcium ions.

TECHNICAL FIELD

The present invention relates to a water-retaining support (or carrier)for plant which can support or hold a plant at the time of the growth ofthe plant and can also function as a source for supplying water to theplant. More specifically, the present invention relates to awater-retaining support for plant which can supply water to a plantwithout inhibiting the growth of the plant, when the support is used asa water-retaining support for fluid seeding (or seeding using a fluid),farm cultivation, field (or bare ground) cultivation, virescence (orgreening) engineering, etc.

The water-retaining support for plant according to the present inventionis also usable in combination with another plant support such as soil soas to enhance the water-retaining ability of the other plant support(i.e., usable as a water-retaining agent) at the time the growth of aplant.

BACKGROUND ART

Polycarboxylic acid-type highly water-absorbing resins, especiallypolyacrylic acid-type polymers, which have been used in a large quantityfor diapers, menstrual goods, etc., are also brought into use in thefield of agriculture due to their inexpensiveness and excellentwater-retaining ability.

For example, hydrogels of the polyacrylic acid-type polymers have beenused as a support for fluid seeding; or a water-retaining support forvirescence engineering, water-saving cultivation, or cultivation onsandy soil, by utilizing their water-retaining ability.

However, it has been recognized that the conventional polyacrylicacid-type hydrogels affect the growth of a plant, and particularly, theycause a marked inhibition of the root origination and root elongationwhen the hydrogels are used in an amount exceeding their appropriateamount (Kazuo Kawashima, et al., “Influences of Highly Water-AbsorbingPolymer Materials on Initial Growth of Crops,” Sand Dune Research,31(1), 1-8, 1984).

Particularly, when the conventional polyacrylic acid-type hydrogel isused as a support for tissue culture, a support for fluid seeding, and asupport for virescence engineering, a plantlet, seed, etc., of a plantare caused to directly contact the high-concentration polyacrylicacid-type hydrogel, and therefore its root origination and rootelongation are markedly inhibited, whereby the use of the polyacrylicacid-type hydrogel is severely restricted. It has also been recognizedthat, in a case where the conventional polyacrylic acid-type hydrogel isused as a water-retaining support for farm or field cultivation, theelongation of the root is inhibited when the concentration of thepolymer in the vicinity of the root is increased so as to enhance theeffect of the water-retaining support.

As an example of the phenomenon such that the above-mentioned hydrogelcomprising a polyacrylic acid-type resin markedly inhibits the growth ofa plant, there has been reported an experiment wherein distilled waterwas absorbed into a crosslinked sodium polyacrylate so as to form ahydrogel, and the thus obtained hydrogel was caused to contact seeds ofcucumbers and kidney beans for respective periods of time (3, 6, 9, 12,24 and 48 hours), and then the states of the germination and rootorigination of the seeds were observed (Kazuo Kawashima, et al.,“Influences of Highly Water-Absorbing Polymer Materials on InitialGrowth of Crops,” Sand Dune Research, 31(1), 1-8, 1984).

As a result of such experiments, it has been reported that the growth ofroots was markedly suppressed in the case of cucumber seeds, when theyare caused to contact the hydrogel for 36 to 48 hours, and that theinhibition of root growth was also observed similarly in the case ofkidney beans. Further, it has been reported that theα-naphtylamine-oxidizing ability of the root was markedly reduced whenthe root is caused to contact the hydrogel for 5 hours or more. In thisreport, such growth inhibition and functional hindrance are presumablyattributable to a fact that the plant cannot effectively use the watercontained in the hydrogel.

On the other hand, it has been reported that, when rice seeds were sownon a hydrogel which had been prepared by causing crosslinked sodiumpolyacrylate to absorb water, and then the process of the rootorigination was observed, serious hindrance in the root origination wasrecognized (Yorio Sugimura, et al., “Utilization of HighlyWater-Absorbing Polymer as Virescence Engineering Material,” Techniquesof Virescence Engineering, 9(2), 11-15, 1983). In this report, nohindrance in the root origination was observed when the hydrogel wasdialyzed with tap water, but the recovery of the root growth was notobserved even when the hydrogel was dialyzed with distilled water. Inthis report, it is presumed that, when the hydrogel is washed ordialyzed with a weak electrolytic solution such as tap water, thewater-absorption amount force toward the hydrogel was weakened, and themigration of water from the gel to the root hair is facilitated, therebyto solve the hindrance in the root origination.

It has also been reported an example wherein the elongation of soybeanroot was markedly inhibited in a soil which had been mixed with acrosslinked sodium polyacrylate hydrogel, as compared with that in thecase of a polyvinyl alcohol-type hydrogel (Tomoko Nakanishi, Bioscience& Industry, 52(8), 623-624, 1994). In this reference, this phenomenon ispresumably attributable to a fact that the water in the sodiumpolyacrylate hydrogel is less liable to be utilized for a plant.

As described above, it has heretofore been considered that theinhibition of the growth of a plant in a hydrogel comprising an alkalimetal salt of crosslinked polyacrylic acid is attributable to a factthat the water in the hydrogel is not effectively utilized for theplant.

An object of the present invention is to provide a water-retainingsupport for plant which has solved the above-mentioned problems of thehydrogel water-retaining support encountered in the prior art.

Another object of the present invention is to provide a water-retainingsupport for plant which has a water-retaining ability comparable to thatof the conventional polyacrylic acid-type hydrogel, and does notsubstantially cause an inhibition in root origination or in rootelongation.

Disclosure of Invention

As a result of earnest study, the present inventors have found that theeffect of a hydrogel is too strong to recognize that the inhibition ofthe root elongation is simply attributable to the effectiveness in theutilization of water in the hydrogel.

As a result of further study based on the above discovery, the presentinventors have also found that the calcium ion-adsorbing ability in thehydrogel has an important effect on the inhibition of root originationor the inhibition of root elongation of a plant which is in contact withthe hydrogel.

The water-retaining support for plant according to the present inventionis based on the above discoveries and comprises a hydrogel-formingpolymer having a calcium ion absorption of less than 50 mg per 1 g ofthe dry weight thereof and having a water absorption magnification inion-exchange water (at room temperature; 25° C.) of 100 or more.

Herein, the “water-retaining support” refers to one in a “dry state”unless otherwise noted specifically. As a matter of course, when such asupport is distributed or circulated in an actual market, etc., thesupport may also be in a “hydrogel” state wherein a part or the entiretyof the support retains water therein (the same as in the descriptionappearing hereinafter).

As a result of further study based on the above discovery, the presentinventors have found that there is a case wherein the above-mentioned“calcium ion absorption (amount)” may greatly be affected by the contentof carboxyl groups bonded to the polymer chain of the hydrogel-formingpolymer.

The water-retaining support for plant according to the present inventionis based on the above discovery and is one comprising a hydrogel-formingpolymer having a carboxyl group bonded to the polymer chain thereof, andhaving a content of alkali metal salt or ammonium salt of the carboxylgroup of 0.3 to 2.5 mmol per 1 g of the dry weight of the resin.

According to the present inventors experiments, it has been found that apreferred embodiment of such a hydrogel-forming polymer is one furthercontaining a calcium salt of the carboxyl group.

As a result of experiments as described hereinafter, the presentinventors have found a fact that the conventional hydrogel comprising an“alkali metal salt of crosslinked polyacrylic acid” selectively adsorbsa heavy metal ion, mainly calcium ion. In other words, according to thepresent inventors' experiments, it is presumed that the conventionalcrosslinked polyacrylic acid-type hydrogel adsorbs ions (mainlycomprising calcium ion) in agricultural water (such as well water, tapwater, river water, and lake water) and the plant suffers fromdeficiency of calcium ion; or the hydrogel directly adsorbs ions (mainlycomprising calcium ion) in the plant body from its roots, whereby theplant suffers from deficiency of calcium ion.

The calcium ion is absorbed by a plant in a physicochemical manner. Whenthe liquid surrounding the plant contains calcium ion in a lowconcentration, the calcium ion is not absorbed by the plant but thecalcium ion is often eluted out of the plant. It is considered that, inthe thus caused calcium ion deficiency, the structure of cell membraneis damaged or broken, so that many important functions dependent on themembrane structure, such as cell division, are stopped or retarded,whereby the elongation of root is markedly inhibited in appearance (withrespect to the details of such deficiency of calcium ion, e.g., “Outlineof Plant Nutritional Science,” edited by Kikuo Kumazawa, p. 118, YokendoK. K., 1974, may be referred to).

As shown in Table 1 in “Examples” appearing hereinafter, when thepresent inventors prepared various hydrogels respectively havingdifferent calcium-absorbing abilities and subjected the resultanthydrogels to root origination tests for seeds, marked growth inhibitionswere observed with respect to the roots and stems thereof, when thecalcium ion absorption became 50 mg or more per 1 g of the dry weight ofthe water-retaining support. Thus, according to the present inventors'knowledge, it is presumed that the marked growth inhibition caused bythe conventional hydrogel comprising the metal salt of crosslinkedpolyacrylic acid is not attributable to the property of water in thehydrogel but is attributable to the calcium ion deficiency in the plantcaused by the absorption of calcium ion from the plant by the hydrogel.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic sectional view showing an embodiment of theplant-growing vessel according to the present invention.

FIG. 2 is a schematic perspective view showing an embodiment of theplant-growing sheet according to the present invention.

FIG. 3 is a schematic perspective view showing another embodiment of theplant-growing sheet according to the present invention.

FIGS. 4A and 4B are schematic perspective views showing otherembodiments (partition-type) of the plant-growing sheet according to thepresent invention.

FIG. 5 is a schematic plan view showing a case wherein thepartition-type sheet according to the embodiment of FIG. 4B is used incombination with another vessel.

FIGS. 6A and 6B are schematic plan views showing examples of theembodiment wherein a hydrogel-forming polymer is disposed in the form ofan intermittent layer on a substrate.

FIGS. 7A, 7B and 7C are schematic sectional views showing examples ofthe embodiment wherein a hydrogel-forming polymer is disposed on thesubstrate of a vessel or sheet in the present invention.

FIG. 8 is a schematic sectional view showing an example of the actualembodiment of the plant-growing vessel according to the presentinvention.

FIG. 9 is a schematic perspective view showing an example of the actualembodiment of the plant-growing sheet (partition-type) according to thepresent invention.

FIG. 10 is a schematic plan view showing one division of thepartition-type sheet of FIG. 9 as viewed from the above.

FIG. 11 is a schematic sectional view showing an embodiment wherein asupport and a plant are disposed in the plant-growing vessel of FIG. 8,and water is supplied to the vessel.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinbelow, the present invention will be described in detail withreference to the accompanying drawings as desired.

In the following description, “%” and “part(s)” representing aquantitative proportion or ratio are those based on weight, unlessotherwise noted specifically.

Water-Retaining Support

The water-retaining support according to the present invention comprisesa hydrogel-forming polymer having a calcium ion absorption (amount) ofless than 50 mg per 1 g of the dry weight thereof, and having a waterabsorption magnification in ion-exchange water of 100 (times) or more.In the present invention, the above-mentioned “calcium ion absorption”and “water absorption magnification” in ion-exchange water may suitablybe measured, e.g., by the following method.

Measurement of Calcium Ion Absorbing Amount

1 g of a dried water-retaining support is added to 1 L (liter) ofaqueous calcium chloride solution having a calcium ion concentration of200 mg/L. Then, the resultant mixture is left standing for 2 days (48hours) in a constant-temperature bath (or thermostatic chamber) at roomtemperature (25° C.) while the mixture is stirred occasionally, therebyto cause the water-retaining support to absorb calcium ion while beingswollen. The thus swollen water-retaining support is separated from thesupernatant, and the calcium ion concentration in the remainingsupernatant (excess amount thereof in the above-mentioned aqueouscalcium chloride solution) is quantitatively determined by atomicabsorption spectrometry (A mg/L). On the basis of the thus determinedvalue (A) of the calcium ion concentration, the calcium ion absorptionamount per 1 g of the water-retaining support is obtained by thefollowing formula. At the time of the separation of the supernatant fromthe water-retaining support, there is a possibility that thenon-crosslinked water-soluble polymer is dissolved in the supernatant,and therefore it is preferred to effect separation by ultrafiltrationusing an ultrafilter membrane which can fractionate the molecular weightof about 1,000 to 3,000.

Calcium ion absorption amount per 1 g of water-retaining support(mg/g)=200−A

When the calcium ion absorption amount measured by the above-mentionedmethod is 50 mg or more per 1 g of the dry weight of the water-retainingsupport, calcium ion deficiency is liable to occur in a plant which isin contact with the water-retaining support as shown in Exampleappearing hereinafter. The calcium ion absorption may preferably be 45mg or less, more preferably 40 mg or less. (Measurement of WaterAbsorption Magnification in Ion-exchange Water)

A predetermined amount (W₁ g) of a dried water-retaining support isweighed, then is immersed in an excess amount (e.g., a weight which isat least 1.5 times the expected water-absorption amount of theabove-mentioned water-retaining support) of ion-exchange water (havingan electric conductivity of 5 μS/cm or less), and is then left standingin a constant-temperature bath at room temperature (25° C.) for 2 days(48 hours) whereby the support is swollen. An excess amount of water isremoved by filtration, and thereafter the weight (W₂ g) of thewater-retaining support which has absorbed water to be swollen therewithis measured. Then, the water absorption magnification is determined bythe following formula:

water absorption magnification=(W ₂ −W ₁)/W ₁

If the water absorption magnification measured by the above-mentionedmethod is less than 100, it becomes difficult to sufficiently supplywater to a plant when a predetermined amount of the water-retainingsupport is used. The water absorption magnification may preferably be140 or more, more preferably 160 or more.

When the salt concentration is relatively low as in the case ofagricultural water, the means for most effectively improving the waterabsorption magnification of a hydrogel is to introduce a dissociativeion group into the gel so as to expand the molecular chains in the geland to simultaneously enhance the internal osmotic pressure in the gel.

Hydrogel-forming Polymer

The hydrogel-forming polymer constituting the water-retaining supportaccording to the present invention refers to a polymer having acrosslinked or network structure, and has a property such that itretains water in the inside thereof on the basis of such a structure soas to form a hydrogel. Further, the “hydrogel” refers to a gel which atleast comprise a crosslinked or network structure comprising a polymer,and water (as a dispersion liquid) retained by such a structure.

The “dispersion liquid” retained in the crosslinked or network structureis not particularly limited, as long as it is a liquid comprising wateras a main or major component. More specifically, the dispersion liquidmay for example be either of water per se, an aqueous solution and/orwater-containing liquid (e.g., a mixture liquid of water and amonohydric or polyhydric alcohol).

In the present invention, it is preferred to use a product obtained bycrosslinking a water-soluble or hydrophilic polymer compound, as theabove-mentioned hydrogel-forming polymer. Such a crosslinked polymer hasa property such that it absorbs water in an aqueous solution to beswollen, but is not dissolved therein. The water absorption rate may bechanged by changing the kind of the above-mentioned water-soluble orhydrophilic polymer and/or the density (or degree) of crosslinkingthereof.

When the aqueous solution of the above-mentioned hydrophilic polymercompound has a cloud point of 70° C. or below, it is possible to obtaina hydrogel-forming polymer such that it shows a decrease in the waterabsorption magnification thereof along with an increase in a temperaturerange of not lower than 0° C. and not higher than 70 ° C., and the waterabsorption magnification of the polymer is reversibly changeable withrespect to temperature.

Water-soluble or hydrophilic polymer compound

Specific examples of the water-soluble or hydrophilic polymerconstituting the water-retaining support according to the presentinvention may include: methyl cellulose, dextran, polyethylene oxide,polypropylene oxide, polyvinyl alcohol, poly N-vinyl pyrrolidone, polyN-vinyl acetamide, polyvinyl pyridine, polyacrylamide,polymethacrylamide, poly-N-acryloyl piperidine, poly-N-n-propylmethacrylamide, poly-N-isopropyl acrylamide, poly-N,N-diethylacrylamide, poly-N-isopropyl methacrylamide, poly-N-cyclopropylacrylamide, poly-N-acryloyl pyrrolidine, poly-N,N-ethyl methylacrylamide, poly-N-cyclopropyl methacrylamide, poly-N-ethyl acrylamide,poly-N-methyl acrylamide, polyhydroxymethyl acrylate, polyacrylic acid,polymethacrylic acid, polyvinylsulfonic acid, polystyrenesulfonic acidand their salts, poly-N,N-dimethylaminoethyl methacrylate,poly-N,N-diethylaminoethyl methacrylate, poly-N,N-dimethylaminopropylacrylamide, and their salts, etc.

Crosslinking

As the method of imparting or introducing a crosslinked structure to apolymer, there are a method wherein a crosslinked structure isintroduced into the polymer at the time of the polymerization of themonomer for providing the polymer; and a method wherein a crosslinkedstructure is introduced to a polymer after the completion of thepolymerization of the monomer. Each of these methods may be used in thepresent invention.

The former method (i.e., introduction of crosslinking at the time ofmonomer polymerization) may generally be conducted by utilizing thecopolymerization with a bifunctional monomer (or a monomer having threeor more functional groups). For example, such a method may be conductedby using a bifunctional monomer such as N,N-methylene bis-acrylamide,hydroxyethyl dimethacrylate, and divinylbenzene.

The latter method (i.e., introduction of crosslinking after monomerpolymerization) may generally be conducted by forming a crosslinkbetween molecules by utilizing light, electron beam, γ-ray irradiation,etc.

Further, the latter method may also be conducted by crosslinking apolymer, e.g., by using, as a crosslinking agent, a multi-functionalmolecule having a plurality of functional groups (such as isocyanategroup) which is capable of being bonded to a functional group (such asamino group) in the polymer.

In the present invention, the above-mentioned water absorption rate ofthe hydrogel-forming polymer is dependent on the above-mentionedcrosslinked structure, particularly the density of crosslinking of thepolymer. In general, as the crosslinking density becomes lower, thewater absorption rate tends to be increased.

In the former method, the crosslinking density may arbitrarily becontrolled, e.g., by changing the copolymerization ratio of thebifunctional monomer. In the latter method, the crosslinking density mayarbitrarily be controlled, e.g., by changing the quantity of irradiationsuch as light, electron beam, and γ-ray.

In the present invention, the crosslinking density may preferably be inthe range of about 0.02 mol % to 10 mol %, more preferably about 0.05mol % to 4 mol %, in terms of the ratio of the moles of the branchingpoint to the moles of all the monomer. Alternatively, when thecrosslinked structure is introduced by the former method (introductionof crosslinking at the time of polymerization), the crosslinking densitymay preferably be in the range of about 0.03 wt.% to 3 wt.%, morepreferably about 0.05 wt.% to 1.5 wt.%, in terms of the copolymerizationweight ratio of the bifunctional monomer to all the monomers (inclusiveof the bifunctional monomer per se).

When the crosslinking density exceeds about 10 mol %, the waterabsorption magnification of the hydrogel-forming polymer according tothe present invention is decreased, whereby the effect of thehydrogel-forming polymer as the water-retaining support is decreased. Onthe other hand, when the crosslinking density is below about 0.02 mol %,the hydrogel-forming polymer becomes mechanically weak, and the handlingthereof becomes difficult.

The crosslinking density (molar ratio of the branching points withrespect to all the monomer) may be determined quantitatively, e.g., by¹³C-NMR (nuclear magnetic resonance absorption) measurement, IR(infrared absorption spectrum) measurement, or elemental analysis.

Further, in the hydrogel-forming polymer constituting thewater-retaining support according to the present invention, it is alsopossible to obtain a better balance between a high water absorptionmagnification and a high mechanical strength in the hydrogel-formingpolymer by making the crosslinking density higher in the vicinity of thesurface than that in the inside thereof (i.e., by introducing so-called“surface crosslinking”). In such an embodiment, the portion having arelatively high crosslinking density in the vicinity of the surface maymainly contribute to the high mechanical strength (and to an improvementin the non-stickiness between support particles), while the portionhaving a relatively low crosslinking density in the inside may mainlycontribute to the high water absorption magnification. Thus, it becomeseasy to realize a preferred mechanical strength and a preferrednon-stickiness between the particles substantially without decreasingthe water absorption magnification.

In view of the balance between the water absorption magnification andmechanical strength, the ratio (Ds/Di) of the highest crosslinkingdensity Ds in the vicinity of the surface to the lowest crosslinkingdensity Di in the inside of the particle in the above-mentionedembodiment may preferably be about 2 to 5, more preferably about 5 to 10(particularly, about 10 to 100).

The crosslinking density in the vicinity of the surface and that in theinside of the particle may be measured by determining the ratio of thepresence of the crosslinking agent in the vicinity of the surface andthat in the inside of the particle, e.g., according to a local analysistechnique such as electron spectroscopy for chemical analysis ESCA(XPS), electron probe microanalysis EPMA, attenuated total reflection(ATR), or secondary ion mass spectrometry SIMS (time-of-flight SIMS(TOF-SIMS), etc.).

In the water-retaining support for plant according to the presentinvention, when the hydrogel-forming polymer constituting the supporthas a high mechanical strength, it becomes easy to keep appropriatevoids (or cavities) between the individual support particles, and thepresence of the voids may further improve the capability of the supportto supply oxygen to the root of a plant.

In the present invention, the method of introducing the surfacecrosslinking to the hydrogel-forming polymer is not particularlyrestricted, and it is possible to use, e.g., various kinds of knownmethods (or a combination of two or more of such methods).

Particularly, when the hydrogel-forming polymer in the present inventionhas a carboxyl group bonded to the polymer chain thereof, it ispreferred to use a method wherein a crosslinking agent having at leasttwo functional groups capable of reacting with the carboxyl group isused to crosslink a portion in the vicinity of the surfaces of finepolymer particles. Examples of such a crosslinking agent may include:epoxy compounds such as ethylene glycol diglycidyl ether (JP-A (JapaneseLaid-Open Patent Application No.) Sho 57-44627); polyhydric alcoholssuch as glycerin (JP-A Sho 58-180223); poly-(or polyvalent) aminecompounds, poly-aziridine compounds, or poly-isocyanate compounds (JP-ASho. 59-189103); poly-epoxy compounds having an amino group (JP-A Sho.63-195205); a reaction product of epihalohydrin and a low-molecularprimary amine such as ammonia or ethylene diamine (JP-A Hei 2-248404);poly-azithidinium base compounds (JP-A Hei 6-287220), etc.

When the molecular weight of the above crosslinking agent is low, thecrosslinking agent is liable to penetrate into the inside of thehydrogel-forming polymer, and there is a case wherein the crosslinkinghas a strong tendency to reach the inside thereof without stopping atthe vicinity of the surface. From such a viewpoint, the molecular weightof the crosslinking agent may preferably be at least 1,000, morepreferably within the range of 10,000 to 100,000, in terms ofweight-average molecular weight,.

As the technique for crosslinking the surface of a hydrogel-formingpolymer with the above crosslinking agent, it is possible to use amethod wherein a hydrogel-forming polymer to be surface-crosslinked isdispersed in a large amount of a low-boiling point organic solvent suchas alcohol, ketone and ether containing water, and then a crosslinkingagent is added to the resultant mixture, thereby to effect crosslinking(JP-A Sho. 57-44627); a method wherein a crosslinking agent is added toa hydrogel-forming polymer containing water wherein the water content isadjusted to 10 to 40 wt.% thereby to effect crosslinking (JP-A Sho.59-62665); a method wherein a crosslinking agent and water are absorbedinto a hydrogel-forming polymer in the presence of inorganic powder, andthe resultant mixture is heated under stirring, so as to simultaneouslyeffect crosslinking and removal of water (JP-A Sho. 60-163956); a methodwherein 1 wt. part of a hydrogel-forming polymer is dispersed into alarge amount of a hydrophilic inactive solvent having a boiling point of100° C. or higher, in the presence of inactive inorganic powder and 1.5to 5.0 wt. parts of water, thereby to effect crosslinking (JP-A Sho.60-14745); a method wherein a hydrogel-forming polymer is treated with acrosslinking agent and an aqueous solution containing any of an alkyleneoxide adduct of monohydric alcohol, a monovalent salt of organic acid,and a lactam, thereby to effect reaction (JP-A Hei 7-33818); etc.

Amount of Residual Organic Material in Polymer

In view of suppression of an adverse effect (such as growth inhibition,necrosis of root tip, and leaf withering) on a plant to be grown byusing the water-retaining support according to the present invention,the amount of an organic material remaining in the above-mentionedhydrogel-forming polymer may preferably be as small as possible. Morespecifically, the total amount of organic materials (reductivematerials) may preferably be 15 ppm or less, more preferably 10 ppm orless (particularly, 5 ppm or less), in terms of the value of chemicaloxygen demand (COD) due to all the organic materials remaining in theliquid which has been obtained by subjecting the polymer to extractionwith distilled water in an amount of 1,000 times that of the polymer.The COD value may preferably be measured, e.g., by the following“potassium permanganate method.”

Amount of Residual Free Carboxylic Acid (or Carboxylate) in Polymer

The amount of free (volatile) carboxylic acid (or carboxylate), such asacetic acid (or acetate), remaining in 1 g of the dry weight of thedried hydrogel-forming polymer used in the present invention maypreferably be 0.5 mmol or less, more preferably 0.3 mmol or less(particularly, 0.1 mmol or less). This “carboxylic acid” may preferablybe measured, e.g., by the following “steam distillation method.”

Potassium Permanganate Method

1 g of the dried water-retaining support is immersed in 1000 g ofdistilled water, and left standing in a constant-temperature bath understirring for 2 days (48 hours) at room temperature (25° C.) so as toextract the organic material (reductive material) remaining in the abovewater-retaining support. 100 ml of the resultant supernatant iscollected from this mixture, and 5 ml of 9N-sulfuric acid and 20 ml ofan N/80 potassium permanganate solution are added thereto. After theresultant mixture is boiled for 5 minutes, 20 ml of N/80 oxalic acidsolution is added thereto, and the excess of the oxalic acid is titratedby using an N/80 potassium permanganate solution (B ml). The chemicaloxygen demand (COD) is calculated by the following formula:

COD (ppm)=B

Steam Distillation Method

1 g of the dried water-retaining support is immersed in 1000 g ofdistilled water, and is left standing in a constant-temperature bathunder stirring for 2 days (48 hours) at room temperature (25° C.) so asto extract the free carboxylic acid (carboxylate) remaining in the abovewater-retaining support. 100 ml of the supernatant is collected from theresultant mixture, 10 ml of 85% phosphoric acid is added thereto, andthe resultant mixture is subjected to steam distillation. The resultantdistillate is titrated by using a 0.01N-aqueous sodium hydroxidesolution (C ml) while using phenolphthalein as an indicator. The free(volatile) carboxylic acid (carboxylate) remaining in 1 g of the driedwater-retaining support is determined as C/10 (mmol).

Polymer Having Carboxyl Group

Examples of an embodiment of the hydrogel-forming polymer having acalcium ion absorption suitable for retaining water for a plant and alsohaving a preferred water absorption magnification in ion-exchange watermay include, e.g., a hydrogel-forming polymer having a carboxyl groupbonded to the polymer chain thereof wherein the polymer chain iscrosslinked, and the content of an alkali metal salt or ammonium salt ofthe carboxyl group is 0.3 to 2.5 mmol per 1 g of the polymer. Thecontent of the alkali metal salt or ammonium salt of carboxyl group maypreferably be 0.5 to 2.0 mmol (particularly, 1.0 to 1.5 mmol). Such apolymer having a carboxyl group may also preferably have theabove-mentioned amount of residual organic material and/or the amount ofcarboxylic acid. The content of the alkali metal salt of the carboxylgroup may preferably be measured, e.g., by the following method.

Method of Measuring Content of Carboxyl Group Salt

0.2 g of the dried water-retaining support is weighed in a platinumcrucible, is subjected to ashing in an electric furnace, and thereafterthe support is dissolved in 5 ml of 1N-hydrochloric acid. Then,distilled water is added to the resultant mixture so as to provide atotal volume of 50 ml, and the cation concentration (D mM) therein isdetermined according to atomic absorption spectrometry. The content ofcarboxyl group salt in 1 g of the dried water-retaining support iscalculated as D/4 (mmol).

The conventional hydrogel comprising crosslinked product of an alkalimetal salt of polyacrylic acid has a water absorption magnificationwhich is markedly higher than that of a hydrogel comprising acrosslinked product of a nonionic hydrophilic polymer, and has been usedas a water-retaining support in the agricultural field because of such ahigh water absorption magnification. However, according to the presentinventor's experiments, in the hydrogel comprising the crosslinkedproduct of the alkali metal salt of polyacrylic acid which hasconventionally been developed as one to be used for agriculture, thecontent of the introduced dissociative ion groups is very high (e.g.,the amount of the introduced alkali metal salt of acrylic acid is about6 mmol or more per 1 g of the dried resin), whereby the hydrogel has atendency such that it adsorbs heavy metal ions such as calcium ion whichare essential for the growth of a plant, and it markedly inhibits thegrowth of the plant, as described above.

In contrast thereto, according to the present inventors' experiments, ithas been found that when 0.3 to 2.5 mmol of a dissociative ion group(e.g., alkali metal salt or ammonium salt of carboxyl group) isintroduced into a water-retaining support per 1 g of the dried support,the support shows a water-retaining effect (water absorptionmagnification in ion-exchange water of 100 or more) which is sufficientfor growing a plant without causing deficiency of calcium ion in theplant.

Here, the alkali metal salt or ammonium salt is preferred as thedissociative ion group, and sodium salt or potassium salt is preferredas the alkali metal salt. In view of the effect on the plant, it ispreferred to use a potassium salt or an ammonium salt which can beabsorbed by the plant as an essential nutrient. When the content of thealkali metal salt of carboxyl group is less than 0.3 mmol per 1 g of thedried water-retaining support, it is difficult for the water-retainingsupport to have a water absorption magnification of 100 or more. On theother hand, when the content of alkali metal salt of carboxyl groupexceeds 2.5 mmol, the calcium ion absorption is liable to become 50 mgor more per 1 g of the dried water-retaining support

Monomer

The hydrogel-forming polymer may be obtained, e.g., by the ternarypolymerization of a monomer (I) having an alkali metal salt or ammoniumsalt of carboxyl group, a hydrophilic monomer (II), and a crosslinkingmonomer (III).

Specific examples of the monomer (I) may include alkali metal salts orammonium salts of acrylic acid, methacrylic acid, maleic acid, itaconicacid, etc. These monomers may be either polymerized as a salt ofmonomer, or polymerized as a carboxylic acid monomer and then convertedinto a salt thereof by neutralization after the polymerization. However,the content thereof may preferably be set to 0.3 to 2.5 mmol per 1 g ofthe water-retaining support

Specific examples of the hydrophilic monomer (II) may include acrylicacid, methacrylic acid, maleic acid, itaconic acid, acrylamide,methacrylamide, N-vinylacetamide, etc. When a monomer containing acarboxylic acid is used as the hydrophilic monomer (II), the resultanthydrogel has a tendency to have a low pH value. Accordingly, in thiscase, the alkali metal salt or ammonium salt content of the carboxylgroup may preferably be set to 1.0 to 2.5 mmol per 1 g.

In such a case, it is also possible to convert a portion of the monomercontaining the carboxylic acid into calcium salt so as to becopolymerized. According to the present inventors' investigation, it hasbeen found that such a calcium salt-type monomer shows an effect ofdecreasing the calcium ion absorption of the water-retaining support, aneffect of avoiding a decrease in pH, and further an effect ofaccelerating the polymerization.

Specific examples of the crosslinking monomer (III) may includeN,N′-methylene bis(meth)acrylamide, N,N′-ethylene bis(meth)acrylamide,ethylene glycol di(meth)acrylate, and diethylene glycoldi(meth)acrylate, etc. The amount of the crosslinking monomer (III) tobe used may generally preferably in the range of 0.01 to 5 mol %, morepreferably in the range of 0.1 to 1 mol % with respect to all themonomers (while somewhat depending on the concentration for thepolymerization). When the amount of the monomer to be used is less than0.01 mol %, the strength of the water-retaining support tends to becomeinsufficient. On the other hand, when the amount of the monomer to beused exceeds 5 mol %, it becomes difficult for the water-retainingsupport to have a water absorption magnification of 100 or more.

It is also possible to obtain the hydrogel-forming polymer by thesaponification of a copolymer comprising vinyl acetate and maleicanhydride, a copolymer comprising vinyl acetate and acrylic acid(acrylate), etc. The thus obtained polymer compound is a polyvinylalcohol-type polymer. When such a polymer is prepared so as to provide acontent of alkali metal salt or ammonium salt of the carboxyl groupbonded to the polymer of 0.3 to 2.5 mmol per 1 g of the dry weight, itis possible to obtain a water-retaining support according to the presentinvention having a calcium ion absorption of less than 50 mg per 1 g ofthe water-retaining support and having a water absorption magnificationin ion-exchange water of 100 or more.

Treatment With Calcium Ion

The hydrogel-forming polymer may also be obtained by treating acommercially available polyacrylate-type highly water-absorbing resinwith a strong acid or calcium ion. In general, in the commerciallyavailable polyacrylate-type highly water-absorbing resin, at least ahalf of the carboxyl groups bonded to the polymer chain are in the stateof alkali metal salts, and the content thereof is at least about 6 mmolper 1 g of the resin. Therefore, the calcium ion absorption per 1 g ofthe resin becomes 120 mg or more, and therefore is inappropriate as thewater-retaining support for a plant.

In the present invention, when the hydrogel-forming polymer containscalcium salt of carboxyl group, the calcium salt content may preferablybe at least 0.1 mmol (more preferably about 1.0 to 3.0 mmol) per 1 g ofthe dry weight of the hydrogel-forming polymer. Such a content of thecalcium salt of carboxyl group may preferably be measured, e.g., by thefollowing method.

Method of Measuring Content of Carboxyl Group Calcium Salt

0.2 g of the dried water-retaining support is weighed in a platinumcrucible, is subjected to ashing in an electric furnace, and thereafterthe support is dissolved in 5 ml of 1N-hydrochloric acid. Then,distilled water is added to the resultant mixture so as to provide atotal volume of 50 ml, and the calcium concentration (E mM) therein isdetermined according to atomic absorption spectrometry. The content ofcarboxyl group calcium salt in 1 g of the dried water-retaining supportis calculated as E/2 (mmol).

When a strong acid such as hydrochloric acid, nitric acid and sulfuricacid, or an aqueous calcium ion solution such as calcium chloridesolution and calcium nitrate solution is added to such a commerciallyavailable polyacrylate-type highly water-absorbing resin, the alkalimetal salt of carboxyl group in the highly water-absorbing resin issubstituted by carboxylic acid or calcium salt of carboxyl group.Therefore, when the amount of the strong acid or calcium ion to be addedis appropriately set, the content of alkali metal salt of the carboxylgroup bonded to the polymer may be adjusted to 0.3 to 2.5 mmol per 1 gof the dried water-retaining support, thereby to provide awater-retaining support for plant according to the present inventionhaving a calcium ion absorption of less than 50 mg per 1 g of the dryweight and having a water absorption magnification in ion-exchange waterof 100 or more.

Here, when the carboxyl group is substituting by carboxylic acid, theresultant hydrogel has a strong tendency to become acidic. Accordingly,particularly in this case, the content of alkali metal salt of carboxylgroup may preferably be adjusted to be 1.0 to 2.5 mmol per 1 g of thedried water-retaining support.

pH of Water-Retaining Support for Plant

The pH (hydrogen ion concentration) of conventional water-retainingsupports for plant containing a hydrogel-forming polymer ranges fromneutral to weakly alkaline. According to the present inventors'knowledge, it is presumed that such a phenomenon is attributable to thereaction condition etc., at the time of the synthesis of polymer.

In contrast, the present inventors have found that, even in awater-retaining support containing a hydrogel-forming polymer, ingeneral, the pH thereof may preferably be weakly acidic so as to providean environment suitable for the growth of a plant.

In general, in the case of a hydrogel comprising a polymer having acarboxyl group, it has a tendency such that the amount of the calciumabsorption of the polymer is decreased as the hydrogen ion concentrationin the polymer composition becomes higher (becomes more acidic).Consequently, also in view of the suppression of the adverse effect ofthe calcium ion absorption of the polymer on a plant, it is preferredthat the pH of the water-retaining support for plant according to thepresent invention is in a weakly acidic range.

Further, the hydrogel comprising a polymer having a carboxyl groupusually has a buffer effect as well, and therefore the hydrogelcomprising a polymer having a carboxyl group is advantageous to theretention of a pH value suitable for plant growth, also in view of thebuffer effect.

In general, the pH of the water-retaining support for plant maypreferably be about pH 3 to 6.5 (more preferably about pH 4 to 6),though it may somewhat vary depending on the kind of a plant.Particularly, since the culture liquid for tissue culture is generallyadjusted to pH 5.7 to 5.8, the pH of the hydrogel may preferably be 5.7to 5.8.

In order to decrease the calcium-absorption, it is sufficient to makethe portion of the carboxylic acid type in the carboxyl group largerthan that of the alkali metal or ammonium salt-type thereof. However,when the portion of this acid type is too large, the pH of awater-retaining agent may tend to become too low or the swelling ratioof the water-retaining agent may tend to decrease. It is possible toobviate or diminish the demerit of a decrease in the pH or swellingratio as described above by increasing the ratio of the calcium salt inthe carboxyl group or decreasing the carboxyl group content (increasingthe nonionic portion) in the polymer.

Method of Culturing Plant Using Hydrogel-Forming Polymer

Heretofore, an agar gel has generally been used as a support for tissueculture. However, in this case, a root is grown therein in a state wherewater is excessive and voids are little present, and therefore the rootis elongated in a form which is different from that of the root grown ina farm cultivation step, whereby it is impossible to acclimate the rootin the inside of the culture vessel. In addition, once an agar geldischarges the water content through the evaporation thereof orabsorption thereof by a plant, the gel hardly absorbs water contentagain. Accordingly, the agar gel does not absorb the water constitutingdew drops or the water once released from the gel in the vessel, wherebythe acclamation of the root is adversely affected in some cases.

According to the present inventors' knowledge, it is presumed that sucha problem of the agar gel is attributable to a fact that agar does notabsorb further water after it is converted into a gel state, that agardoes not absorb water again after it releases water and that it retainswater only with a weak attracting force. In the present invention, sucha water-retaining ability of the gel may be represented, e.g., by a pFvalue.

Here, the pF value (Potential of Free Energy: water absorption pressure)is a value representing the water-retaining ability of support. Withrespect to the details thereof, e.g., “Introduction to Soil” (YasuoTakai and Hiroshi Miyoshi, Asakura Shoten, 1977, pp. 88-89) can bereferred to.

In the present invention, the water which is absorbable by a plant maypreferably be one having a pF value not higher than a capillaryconnection breaking point (pF value of about 2.8). Further, the pF valuemay preferably be not higher than 2.3 so that a plant is preferablygrown in farm cultivation. The water having a pF value of 1.8 or less(gravitational water) can be absorbed by a plant, but it tends to flowout from a vessel having an open-type basement portion. In the case of avessel having a closed-type or closed-like basement, the gravitationalwater may reside in the support at the basement of the vessel, therebyto cause root decay in some cases. In general tissue culture for auseful plant, the vessel is formed into a close system, and an agar gelis used as a support therefor, whereby the pF value during the culturingperiod of time becomes substantially almost zero.

On the other hand, when a hydrogel-forming polymer does not reach itsequilibrium water absorption, the polymer tends to absorb watersurrounding gel particles. In the present invention, when ahydrogel-forming polymer is used as a culturing support, the watercontained in the gel is gradually decreased (pF value thereof isincreased) due to the evaporation of water toward the outside of thevessel during the culturing process and the water absorptionaccompanying the growth of a plant, whereby the acclimation to waterstress may be automatically effected during the culturing process. Inaddition, when the hydrogel-forming polymer is in form of a gelparticle, the voids which are present outside of the gel particles arewidened along with an increase in the pF value, whereby the amount ofoxygen supply may be increased along with the growth of the plant.

Further, when the hydrogel-forming polymer is used in the presentinvention, the water in the form of dew drops in a vessel or the waterseparated from the polymer gel (which often adversely affects theacclimation of root) can also be absorbed by the hydrogel-formingpolymer. Therefore, when the hydrogel-forming polymer is used as asupport, the root may automatically be acclimated in a culturing stepalong with the growth of a plant, whereby the plant may favorably begrown even after the transferring thereof into a farm cultivation step.

Another advantage to be obtained in a case using a hydrogel-formingpolymer as a support for tissue culture is that the space in the vesselmay fully be utilized. The physical environment in a plant support canbe divided or classified into three phases including a liquid phase, agaseous phase, and a solid phase, and the hydrogel-forming polymerfunctions as both of the liquid and solid phases, thereby to secure alarge amount of nutrient and water per unit volume.

Still another advantage to be obtained in a case using thehydrogel-forming polymer as a support for tissue culture is thatadditional culture liquid may easily be added to the support in thecourse of the culturing. In this case, the hydrogel-forming polymer maybe caused to absorb the thus supplied culture liquid without sinking theplant in the culture liquid.

Voids in Support Comprising Hydrogel-Forming Polymer

When the strength of the hydrogel-forming polymer is low, the resultantgel tends to be deformed, thereby to reduce the voids among the gelparticles. Therefore, it is possible to secure the voids by mixing aporous material such as pearlite with the gel. It is also possible toform voids among the gel particles, by increasing the strength of thehydrogel-forming polymer. For the purpose of enhancing the gel strength,it is possible to increase the crosslinking density or impart surfacecrosslinking to the gel.

In this case, known materials such as pearlite, bark, sponge, andsphagnum may be used without any particular restriction. In view of moreeffective exhibition of a bacteriostatic or fungistatic property (seeJapanese Patent Application No. 6-139140; and PCT/JP95/01223) which is acharacteristic of the hydrogel-forming polymer, it is preferred to usean inorganic porous material such as pearlite, as compared with the useof a natural organic matter which is decayable.

(Method of Utilizing Hydrogel-Forming Polymer in Suspension Culture

The hydrogel-forming polymer is also preferably usable in liquid culture(or suspension culture). In the conventional liquid culture, there havebeen posed problems such as the collision of cell agglomerates with thewall surface or collision between the cell agglomerates at the time whenplantlets are being stirred during the liquid culture; and a decrease inthe growth (or multiplication) rate of plant cells caused by a browningmaterial produced by the cells due to the above physical damage.

In contrast, in the present invention, when hydrogel-forming polymerparticles are mixed into a suspension culture system in an extentwherein a liquid state can be maintained, the gel particles may act as acushion, thereby to suppress the production of the browning material, toenhance the growth rate, and to enlarge the cell agglomerate.

The ratio of the volume of the hydrogel to that of the liquid maypreferably be 0.5 to 90%, more preferably 1 to 60%, and particularlypreferably 5 to 40%.

Examples of the suspension culture may include the rotary shake culturewith Erlenmeyer flasks, fermenter culture, large-size tank culture, etc.

Seed Germination and Germination Activity Test

In order to evaluate the effect of a water-retaining support upon aplant, it is preferred to conduct a germination and germination activitytest for a seed by using, as a culture medium, the water-retainingsupport (hydrogel) which has absorbed agricultural water therein. Forexample, seeds of white radish sprouts (e.g., those sold by Takii ShubyoK. K.) which may easily be subjected to short-term germination andgermination activity test may be used as a seed material, and syntheticwater having a typical underground water composition (Table 2) may beused as the agricultural water in the above-mentioned test.

For example, the seed germination and germination activity test may beperformed in the following manner.

16 ml of the above-mentioned synthetic water and 160 mg (1 wt. %) ofeach kind of water-retaining support are introduced into a test tube(having a diameter of 2.5 cm and a height of 15 cm), and the resultantmixture is fully stirred, and then the mixture is left standing for 30minutes at 25° C., thereby to prepare a gel-like culture mediumcomprising the water-retaining support which has absorbed theagricultural water therein. 5 grains of the above-mentioned seed ofwhite radish sprouts are uniformly put on the surface of the gel-likeculture medium in each of test tubes, and the test tube is capped with asilicone plug having a 6-mm diameter hole filled with cotton. The thuscapped test tube is cultured for 4 days in a culture room (25° C.,illumination intensity of 2000 Lux, 16h-daytime), and the ratio ofgermination (number of germinated seeds/5 (grains)×100(%)) isinvestigated.

In the above-mentioned germination and germination activity test, thecase wherein the seed coat is torn and the cotyledon unfolds is definedas the occurrence of germination, and the other cases are defined as nooccurrence of germination. The length of the above-ground portion ismeasured as the average stem and leaf length from the base portion(branching point between the root and stem) of the germinated individualto its leaf tip, while the length of the underground portion is measuredas the average root length from the base portion of the germinatedindividual to the tip of its main root. Further, the appearance of theroot tip, etc., is observed.

Method of Using Water-Retaining Support

The water-retaining support according to the present invention may beused either singly or in combination with another plant-growing supportas desired. The kind, ratio of amount to be used, etc., of the otherplant-growing support are not particularly restricted. Preferredexamples of the other plant-growing support may include: soil or gravel,sand, pumice, carbide, peat, vermiculite, bark, pearlite, zeolite, rockwool, sponge, sphagnum, crushed coconut shell, crypto-moss, etc. Each ofthese plant-growing supports may be used either singly or in acombination of two or more species thereof, as desired.

When a plant is grown by using the water-retaining support according tothe present invention, the water-retaining support according to thepresent invention comprising a hydrogel or polymer may preferably bemixed with the above-mentioned other plant-growing support comprisingsoil, etc., at a mixing ratio of about 0.1 to 10 wt. % (more preferablyabout 0.3 to 3 wt. %) in terms of weight percent in a dried state.

When the water-retaining support according to the present invention andthe other plant-growing support are used in combination, they may beused as the above-mentioned mixture, and may also be used in anembodiment wherein at least one layer comprising the water-retainingsupport according to the present invention may be disposed on thesurface of and/or in the inside of the other plant-growing support.

Method of Cultivating Plant Using Hydrogel-Forming Polymer

In the conventional cultivation using an open-type vessel (such as pot,cell tray, and planter), the amounts of water and the nutrientconcentration are drastically changed before and after the watering,whereby it is difficult to control water. Immediately before thewatering, the obstruction to the root due to the high concentration ofthe fertilizer in the soil caused by drying becomes problematic, and thewilting of a plant due to the shortage of water becomes problematic. Onthe other hand, immediately after the watering, the residence orretention of excess water in the pot, and the root decay due to theshortage of oxygen become problematic. Particularly, in view of theextreme increase in the fertilizer concentration immediately before thewatering, it is necessary to set the absolute amount of the fertilizerto a low level so as to avoid the extreme increase thereof, and such alow level may cause the suppression of the inherent growth of the plant.

The production of plantlets with a cell-type partition such asvegetables for which the demand has drastically been increasing inrecent years, also holds the above-mentioned problem. In the case ofsuch plantlets with a cell-type partition, since each cell or divisionhas a relatively small volume, and therefore the nutrient concentrationand water content are liable to be changed drastically, thereby to makeit difficult to uniformly control the individual cells.

In the present invention, such a problem of the physical environmentaround the plant root may be represented by the above-mentioned pF value(water absorption pressure). A plant may absorb water having a pF valueof about 2.8 or less, but water having a pF value of 2.3 or less ispreferred in view of favorable growth of a plant. Water having a pFvalue of 1.8 or less can be absorbed by a plant, but it is gravitationalwater and has a strong possibility of flowing out of the rhizosphere (orzone under the influence of the root). On the other hand, when thedrainage of the rhizosphere is poor, the water may reside around theroot, thereby to cause the root decay.

According to the present inventors' knowledge, it is presumed that theroot of a plant which has been cultured by the conventional method isnot acclimated, and the new support (such as bark) to be used in thestep of farm cultivation and the root do not sufficiently fit with eachother (the contact area therebetween is small), whereby the absorptionof the nutrient and water necessary for the initial growth of the plantis insufficient. According to the present inventors, it is also presumedthat the decrease in the germination ratio of seeds and the growthinhibition after the germination are caused by the small contact areabetween the support and the seed or the root after the germination. Whena cultured plantlet with a root is transferred to farm cultivation, theconventionally used support is too hard or does not have a fluidity,thereby to damage the root. In addition, when the conventional supportis used, it is impossible to use the insertion transplantationtechnique.

On the other hand, when the hydrogel-forming polymer according to thepresent invention is used, a large amount of water may be secured perunit volume, whereby the range of fluctuation in the nutrientconcentration in a vessel becomes small and the inhibition of plantelongation is dramatically decreased. Further, since thehydrogel-forming polymer according to the present invention maycompletely absorb an excess of water, the root is less liable to decay,and the control of the nutrient and water becomes easier. Particularly,since the range of fluctuation in the nutrient concentration before andafter the watering is decreased, the absolute amount of a fertilizer maybe increased drastically, thereby to further accelerate the growth ofthe plant. Therefore, according to the present invention, it becomeseasy to uniformly control the individual cells even in the case of theproduction of the plantlets with a cell-type partition wherein thevessel has a relatively small volume.

In addition, a plant immediately after the transplanting, a seed, and aroot after germination may more easily fit with the hydrogel-formingpolymer (the contact area therebetween is increased), thereby tosmoothly conduct the initial growth of the plant. Further, since thehydrogel comprising the hydrogel-forming polymer according to thepresent invention is relatively soft and has a good fluidity, a root maybe transplanted therein without being damaged. Due to such acharacteristic of the hydrogel, the insertion transplantation becomeseasier. While such genera of orchids as Phalaenopsis and Cymbidium,particularly, have thick roots with substantially no root hair, thepresent invention makes it very easy to transplant such plant species aswell.

Method of Preventing Flowing-out of Support

When a vessel is one having an open-type basement, it is important toprevent the flowing-out of a support (such as hydrogel and plantingmaterial including the hydrogel) due to watering, etc. As the means forpreventing such flowing-out, it is effective to enlarge the particles ofthe hydrogel-forming polymer or increase the stickiness thereof. As amethod of increasing the stickiness, it is possible to use a method ofreducing the crosslinking density of the hydrogel-forming polymer, etc.

Method of Suppressing Rising of Plant

In the case of a plant species with a thick and strong root, when theroot elongates and reaches the bottom face of a vessel, the plant may belifted up to the upper portion of the vessel (so-called “rising”phenomenon). As the means of preventing such a phenomenon, it iseffective to increase the stickiness of the hydrogel-forming polymer. Asthe method of increasing the stickiness, it is possible to use a methodof reducing the crosslinking density of the hydrogel-forming polymer,etc.

Support for Plant Factory

Heretofore, in a so-called “plant factory” (plant-growing system underan artificial environment other than the natural environment such asfield cultivation), cultivation using mist, cultivation using capillarywatering, etc., have been effected, and these methods have required anenormous amount of investment for the watering equipment.

When the hydrogel-forming polymer according to the present invention isused as a plant support or water-supplying medium for such a “plantfactory,” the watering equipment is simplified, thereby to simplify theplant-growing system and reduce the costs therefor.

Field Cultivation

The conventional field cultivation has been encountered with problemssimilar to those in the conventional cultivation using a vessel. Thatis, the field cultivation is affected by conditions of nature, andtherefore the nutrient concentration, water content, and pF value aredrastically changed before and after rainfall, thereby to make itdifficult to cultivate the plant. Particularly, areas with less rainfallhave often been encountered with damages such as drought.

In contrast, when the hydrogel-forming polymer according to the presentinvention is used for the field cultivation, since the hydrogel-formingpolymer functions as a buffer against the drastic fluctuations in thenutrient concentration, water content, and pF value, etc., as describedabove, the plant may be cultivated under a milder condition.

Virescence Technology

with respect to the virescence (or greening) of desert, virescence ofslopes, virescence of wall surfaces, etc., since the basic support issand, soil or clay wall, concrete, etc., the amount of water retainedtherein is very small, and the water-retaining ability thereof is verypoor. For the purpose of smoothly effecting the initial stage of plantgrowth or seed germination from such a state, it is quite effective touse the hydrogel-forming polymer according to the present inventionhaving a very great water-retaining ability and acting as a bufferagainst the drastic fluctuations of the nutrient concentration, watercontent, pF value, etc.

For the virescence of slope, it is possible to sow seeds for virescenceby a fluid seeding method using the hydrogel-forming polymer accordingto the present invention in the same manner as in the technique forspraying a concrete material onto a slope. Particularly, in the case ofthe virescence of a slope or non-flat hillside wherein rocks, etc., areexposed to the ground surface thereof, the attachment ratio of a net andseeds onto the slope tends to become lower when a technique such as netseeding is used. When the fluid seeding method for virescence seed usingthe hydrogel-forming polymer according to the present invention is used,the seeds contained in the hydrogel may uniformly be sprayed onto aslope, and the attachment ratio of the hydrogel and the seeds containedtherein with respect to the slope is increased, thereby to enhance thegermination rate of the seeds and accelerate the growth of the plantafter the germination.

Spatial Cultivation

An epiphyte such as Vanda, which is a genus of orchid family plant, isattached to a tree, etc., in a natural state, while hanging down itsroots into a space, thereby to absorb water of fog, rain, etc. When sucha plant species is artificially cultivated in a space, it is necessaryto increase the frequency of watering so as to prevent drying. In such acase, when the epiphyte is cultivated while covering the periphery ofthe roots thereof with the hydrogel-forming polymer according to thepresent invention, the drying thereof may be prevented for a long periodof time, and the frequency of the watering may be reduced.

Spatial Floating Cultivation Under Weightlessness

With the advent of the age of population growth and food shortage, plantcultivation in the outer space has been under investigation. Since theouter space is weightless, when a plant support mainly comprising ahydrogel-forming polymer is floated in a weightless space such as aspace station, and the support is planted with a plant so as tocultivate the plant, three-dimensional cultivation can be conducted,thereby to drastically increase the plant production per unit volume.

Method of Transplanting Plant

When a plant is transplanted together with the hydrogel-forming polymeraccording to the present invention attached to its roots, the initialdrying may be prevented, thereby to increase the ratio of taking rootand to enhance the initial growth of the plant. Such a transplantingmethod is particularly effective in transplanting plantlets of flowerand vegetable, and woody plantlets, transplanting turf, moving adulttrees, etc.

Method of Shrinking Swollen Hydrogel-Forming Polymer

A hydrogel-forming polymer comprising a polymer having a carboxyl groupin a swollen state (gel state) in water may drastically be shrunk byadding thereto a high concentration of calcium solution or calcium saltpowder. Examples of the use and application of such “gel shrinkage” willbe explained in the following.

(1) When a tissue-cultured plant is transferred to farm cultivation, asugar becomes a cause of germ propagation. Therefore, calcium is addedto a gel to shrink the gel, and the sugar in the gel is removed bydecantation, washing with water, etc.

(2) When a large amount of water is present around a plant such asplantlet at the time of its shipping, the root would be damaged duringthe transportation. Further, the large amount of water makes the goodsheavier, thereby to increase the cost of the transportation. For thepurpose of preventing these problems, calcium is added to a gel so as toshrink the gel, thereby to remove the water in the gel.

(3) In order to increase the contact area between a new support androots when a plant is transplanted, calcium is added to a gel so as toshrink the gel, and thereafter the new support is disposed around theroots thereby to smoothly effect the transplantation.

(4) When a plant grown in a vessel is transplanted, calcium is added tothe gel so as to shrink the gel and to reduce its volume, and water isreleased from the gel, thereby to facilitate the removal of the plantfrom the vessel.

Method of Suppressing Propagation of Algae, etc.

Since algae which have been propagated in the upper portion of a pot,etc., absorbs a nutrient supplied to a plant for the purpose of growingthe plant, it is desirable to suppress the propagation of such algae asfirm as possible. Examples of the suppressing method usable in this caseare as follows:

(1) Covering the surface of the water-retaining support for plant with alight-shielding sheet such as aluminum.

(2) Sprinkling the surface of the water-retaining support for plant withlight-shielding activated charcoal.

(3) Blackening the hydrogel-forming polymer itself by using a pigment,etc.

Additives

In the crosslinked structure of the hydrogel-forming polymerconstituting the plant-cultivating support, soil-improving agent, vesselor sheet according to the present invention, at least water is retainedas desired, so as to form a hydrogel. Further, it is also possible toadd another additive to the hydrogel as desired. As the additive to beincorporated into the inside of the hydrogel or polymer for such apurpose, it is possible to use known additives which may ordinarily beused in the usual plant cultivation in open-air field or facilities(such as greenhouse) without particular limitation.

Specific examples of such a known additive may include: variousnutrients for a plant, agents participating in the cultivation of aplant other than the nutrients (such as plant growth-regulatingsubstance, plant form (or shape)-regulating substance including adwarfing agent) or agricultural chemicals (such as weed killer,insecticide, and bactericide).

Nutrient

Specific examples of the nutrient which may be introduced, as desired,into the inside of the hydrogel or hydrogel-forming polymer according tothe present invention may include major elements such as N, P, K, Ca, Mgand S and/or minor elements such as Fe, Cu, Mn, Zn, Mo, B, Cl and Si.

As the method of incorporating such a nutrient into the hydrogel orhydrogel-forming polymer, it is possible to use a method wherein theabove hydrogel or hydrogel-forming polymer itself is immersed in anaqueous solution containing a substance such as urea, calcium nitrate,potassium nitrate, potassium hydrogen phosphate, magnesium sulfate, andferrous sulfate to be swollen, thereby to cause the resultant hydrogelor hydrogel-forming polymer to absorb thereinto the desired nutrient.

Plant-growth regulating substance, etc.

It is also possible to incorporate into the above-mentioned hydrogel orhydrogel-forming polymer the above-mentioned plant growth-regulatingsubstance, plant form-regulating, etc., or agricultural chemicals (suchas weed killer, insecticide, and bactericide) as desired, which is asubstance participating in the cultivation of the plant other than theabove-mentioned nutrients.

Method of Incorporating Additive

As the method of incorporating one of the above various additives intothe inside of the hydrogel or hydrogel-forming polymer, it is possibleto use a method wherein the hydrogel or hydrogel-forming polymer isimmersed in an aqueous solution of the additive so that the hydrogel orpolymer is caused to absorb the above aqueous solution, thereby toprepare a hydrogel or hydrogel-forming polymer. Further, when a plantform-regulating substance (dwarfing agent) such as inabenfide oruniconazole which has a very low solubility in water is used, it is alsopossible to incorporate the plant form-regulating substance into theinside of the hydrogel or hydrogel-forming polymer by using an organicsolvent which is capable of dissolving the plant form-regulatingsubstance and is capable of swelling the hydrogel or polymer, wherebythe plant form-regulating substance may be incorporated into the insideof the hydrogel or polymer in a practically usable concentration.

Plant Growth in Semi-closed Ecosystem

In the natural world, there works a material circulation ecosystemwherein plants perform photosynthesis, animals eat the plant,microorganisms decompose the excrements of animals and the corpses ofanimals and plant, and the plants absorb the resultant decompositionproducts as nutrients. On the other hand, the crop cultivation consuminga large amount of chemical fertilizers, agricultural chemicals, etc.,may be called a semi-closed ecosystem since the material-circulatingfunction of organisms is suppressed therein. The clonal plantletproduction by aseptic culture and the vegetable production in plantfactories, which have recently been commercialized, may be called aclosed ecosystem since they block up the microorganism phase. It isexpected that the plant cultivation in a closed ecosystem which isindependent of fluctuations in the natural environment and may beartificially controlled, further magnifies its importance in the future.

By utilizing the bacteriostatic action of the hydrogel-forming polymer(see PCT/JP95/01223), the present invention enables plant production ina semi-closed ecosystem wherein a plant is cultivated while the materialcirculation caused by the microorganic decomposition is suppressed. Thismethod has a merit such that not only the propagation of germs such aspathogenic microbes capable of preventing the growth of a plant may besuppressed, but also the oxygen consumption due to microorganisms in thesupport is decreased, thereby to secure a large absolute amount ofoxygen which may be absorbed by the root of the plant. Further, themicroorganism phase may be simplified, such that a plant is grown whileonly the microorganisms effective for the plant (e.g., vasiculararbuscular mycorrhiza) are propagated.

With the advent of the age of population growth and food shortage, theplant production in the outer space has become very important, and theplant production in a closed ecosystem excluding or simplifying themicroorganism phase would prevail in spaces such as space stations. Evenin the plant cultivation in such an outer space, the hydrogel-formingpolymer according to the present invention may preferably be used as asupport for plant.

Plant Cultivation in Home

In order to cultivate a plant in a home, it is particularly importantthat the vessel containing a support may be maintained in a clean stateand that nutrients and water may be supplied easily. Since thehydrogel-forming polymer attached to the vessel according to the presentinvention has a bacteriostatic action (see PCT/JP95/01223), it mayeasily maintain a clean state. Further, since the polymer may retain alarge amount of nutrients and water, the frequency of watering may bereduced, and appropriate nutrient concentration, water content, pFvalue, etc., may be maintained for a long period of time. It is alsopossible to place a plant body such as seed in this vessel from thebeginning advance.

Plant-Growing Vessel/Sheet

Hereinbelow, there is described an embodiment wherein thehydrogel-forming polymer according to the present invention is appliedto a plant-growing vessel or sheet. Such a growing vessel or sheet maypreferably be used for the germination of a seed or growth thereof afterthe germination (hereinafter, the term “growth” is used in a meaningsuch that it also includes germination and growth after the germination)in tissue culture or farm cultivation, and for the growth of a plant.

In this embodiment, the transplanting operation for a plant(hereinafter, the term “plant” is used in a meaning such that it alsoincludes “seed”) may easily be effected, the germination or growth ofthe plant may be accelerated, and the necessity for strict watercontrol, etc., may greatly be alleviated.

The plant-growing vessel in such an embodiment comprises a vessel-shapedsubstrate which is capable of accommodating therein at least a portionof a plant; and a hydrogel-forming polymer disposed in the inside of thevessel-shaped substrate, which has a crosslinked structure.

Further, the plant-growing sheet in such an embodiment comprises asheet-shaped substrate; and a hydrogel-forming polymer disposed on atleast one side of the surface of the substrate, which has a crosslinkedstructure.

In the above-mentioned vessel or sheet according to the presentinvention, the hydrogel-forming polymer having a crosslinked structuremay preferably be a polymer which shows a decrease in water absorptionmagnification along with an increase in temperature within a temperaturerange of not lower than 0° C. and not higher than 70° C., and exhibits awater absorption magnification which is reversibly changeable withrespect to temperature.

Further, in the present invention, when the above hydrogel-formingpolymer is in the form of powder or particles, the powder or particlesmay preferably have a dimension or size of about 0.1 μm to 5 mm in a drystate thereof.

Function of Vessel or Sheet

When the plant-growing vessel or sheet according to the presentinvention is used, the above-mentioned problem encountered in the priorart may be solved on the basis of the function peculiar to the vessel orsheet according to the present invention as described hereinbelow.

More specifically, a polymer capable of providing a hydrogel having acrosslinked structure is disposed on the inner wall of the plant-growingvessel according to the present invention (or on the side of the sheetaccording to the present invention, on which a plant is to be disposed,when such a sheet is disposed on the inner wall of another vessel) bycoating, etc. Accordingly, when a plant is put into the vessel and thenthe vessel is filled with water or a suspension culture medium, theabove-mentioned hydrogel-forming polymer absorbs water so that thevolume thereof is increased remarkably, and occupies the inner space ofthe vessel, whereby the polymer functions as at least a part of thesupport for the plant (in other words, the hydrogel-forming polymerfunctions as such a support, or promotes the function for supporting theplant).

In the present invention, on the basis of the function peculiar to theabove-mentioned which is capable of providing a hydrogel and has acrosslinked structure, the problems encountered in the prior art at thetime of the transplanting of a plant are solved. More specifically, suchproblems to be solved may include: one such that when a plant istransferred into a vessel after the vessel is filled with a solid plantsupport in advance, the root of the plant does not enter the inside ofthe support well, and therefore the resultant workability is decreased,and the root per se is also damaged; one such that when a plant is putinto a vessel and then the conventional solid plant support is chargedinto the vessel, the resultant initial growth is decreased due to asmall contact area between the root of the plant and the support; etc.

In addition, in an embodiment of the present invention wherein thehydrogel-forming polymer to be disposed on the inner wall of the vesselby coating comprises a hydrogel-forming polymer wherein the waterabsorption magnification is decreased along with an increase intemperature in the temperature range of not lower than 0° C. and nothigher than 70° C., and the change in the water absorption magnificationis reversible with respect to temperature, for example, it is possiblethat a plant is put into such a vessel, water or a suspension culturemedium is poured into the vessel so that the polymer is caused to absorbwater, whereby the polymer is swollen so as to occupy the inner space ofthe vessel and the plant is grown by using the hydrogel-forming polymeras (at least a part of) the support of the plant. After the plant isgrown, when the temperature of the support is elevated, thehydrogel-forming polymer is de-swelled (or shrunk) so as to markedlydecrease its volume, and therefore the grown plant may easily be removedfrom the vessel.

Accordingly, the present invention solves the above-mentioned problemsencountered in the prior art, i.e., one such that since the thicklygrown root presses the wall surface of the vessel, a considerable periodof time is required in order to take out the plant from the vessel, andsuch an operation damages the root.

Further, the plant-growing vessel or sheet according to the presentinvention having the above-mentioned structure can solve theabove-mentioned problems on the basis of the function peculiar to such avessel or sheet, as described hereinbelow.

A polymer capable of providing a hydrogel having a crosslinked structureis disposed on the inner wall of the vessel or sheet according to thepresent invention by coating, etc. When the support (such as soil) inthe neighborhood of the inner wall of the vessel assumes awater-excessive state for the above-mentioned reason, the polymerabsorbs water and becomes a hydrogel state. On the other hand, when thesupport in the neighborhood of the inner wall of the vessel assumes awater-deficient state, the hydrogel particles have a function oftransferring water therefrom into the support. As a result, theenvironment for water in the rhizosphere in the neighborhood of theinner wall of the vessel is maintained almost constant, and the problemsencountered in the prior art are solved.

Particularly, in an embodiment of the present invention wherein theabove hydrogel-forming polymer comprises a hydrogel-forming polymerwherein the water absorption magnification is decreased along with anincrease in temperature in the temperature range of not lower than 0° C.and not higher than 70° C., and the change in the water absorptionmagnification is reversible with respect to temperature, the polymerabsorbs water from the support when the temperature becomes lower, whilethe polymer discharges water into the support when the temperaturebecomes higher. In other words, the water content in the support in theneighborhood of the sheet or the wall of the vessel is increased as thetemperature becomes higher. In general, it is considered that a plantdemands a smaller amount of water when the temperature is low (belowabout 5-20° C.), and demands a larger amount of water as the temperaturebecomes higher (about 20-35° C.). It is also considered that theexcessive water content at a low temperature invites a root decayphenomenon, and the deficient water content at a high temperatureinvites growth inhibition. Accordingly, when the above-mentioned vesselor sheet having a hydrogel-forming polymer disposed therein is used, theenvironment in the rhizosphere is maintained more suitably, thereby topromote the growth of the plant more effectively.

In addition, the hydrogel-forming polymer disposed on the inner wall ofthe plant-growing vessel (or on the sheet to be disposed on the innerwall of the vessel) has a function of storing water content and/ornutrients in the crosslinked structure of the polymer as describedabove. Therefore, the storing function which has been performed by the“space” in the conventional growing vessel, may be performed by theabove polymer extremely effectively in place of the above space.Therefore, according to the present invention (even when the ability ofthe growing vessel for storing water content and nutrients is retainedconstant), the internal volume of the vessel can be reduced remarkably.

As described above, according to the present invention, the volume of avessel which has been considered to be “appropriate” in the prior artcan be reduced remarkably, and further the originating power of the rootcan be improved due to an increase in the opportunity for the mechanicalcontact stimulus. Further, on the basis of the reduction in the internalvolume of the vessel per se, it is also possible to reduce the area tobe used for growing a plant, to reduce the amount of the material forthe growing vessel, and to reduce the transporting costs, etc. Inaddition, in combination with the above-mentioned labor saving in thewater control, remarkable cost reduction can be accomplished.

Further, since the conventional vessel for home use has a lower portionof an open-system, and an excess of water is discharged from theopen-system lower portion at the time of the watering, etc., a“receiving pan” must be used simultaneously with the vessel. The use ofsuch a pan is troublesome and it is liable to impair the beautifulappearance of the system.

On the contrary, in the plant-growing vessel according to the presentinvention, since the water-storing ability is imparted to the wallsurface of the vessel, it is not necessarily required to provide anopening portion at a lower part of the vessel. In other words, theopening portion of the vessel is omissible in the present invention.When the vessel having a closed-type lower portion is used, the problemsencountered in the conventional vessel for home use (having anopen-system lower portion) are easily solved.

In the above, the growth of a plant after the germination thereof hasmainly been described, but the vessel or sheet according to the presentinvention is also suitably applicable to the germination of a seed orthe growth thereof after the germination.

Shape of Hydrogel or Hydrogel-forming Polymer

The shape or form of the hydrogel or hydrogel-forming polymer to bedisposed in the inside of the vessel according to the present inventionis not particularly limited, but may appropriately be selected dependingon the kind of a plant, growth method therefor, etc. Specific examplesof the shape of the hydrogel or polymer may comprise various shapes suchas layer-like shape, micro-bead-like shape, fiber-like shape, film-likeshape, and indeterminate shape.

The dimension or size of the hydrogel or polymer in the presentinvention may appropriately be selected depending on the kind of theplant, cultivation method therefor, etc. In order to enhance thewater-absorbing rate for the hydrogel-forming polymer, it is preferredto increase the surface area of the hydrogel or hydrogel-forming polymerper unit volume thereof, that is, to decrease the dimension of oneobject (e.g., one particle) of the hydrogel or hydrogel-forming polymer.For example, the dimension or size of the hydrogel or polymer in thepresent invention may generally be in the range about 0.1 μm to 1 cm,more preferably in the range about 1 μm to 5 mm (particularly about 10μm to 1 mm), in a dried state thereof.

In the hydrogel or polymer according to the present invention, theabove-mentioned “dimension in a dried state” refers to the average ofmaximum diameters (maximum dimensions) of the hydrogel or polymer(average of values obtained by measuring at least 10 objects). Morespecifically, e.g., the following dimension may be treated as the“dimension in a dried state” according to the shape of the abovehydrogel or hydrogel-forming polymer.

Micro-bead shape: particle size (average particle size);

Fiber shape: average of lengths of respective fiber-like pieces;

Film shape, indeterminate shape: average of maximum dimensions ofrespective pieces; and

Layer shape: thickness of a polymer layer.

In the present invention, in place of the above “average of maximumvalues”, it is also possible to use the diameter of a “ball” having avolume equal to the average of the volumes of respective pieces (averageof values obtained by measuring at least 10 pieces) as the “dimension ina dried state” of the particles of the above hydrogel orhydrogel-forming polymer.

Method of Shaping Hydrogel or Polymer

The method of shaping of the hydrogel or hydrogel-forming polymeraccording to the present invention is not particularly limited. AS sucha method, it is possible to use an ordinary method of shaping a polymerdepending on the desired shape of the hydrogel or polymer.

When the simplest method is used, a monomer for providing thewater-soluble or hydrophilic polymer, the above-mentionedmulti-functional monomer (such as bifunctional monomer), and apolymerization initiator are dissolved in water, and the monomer, etc.,is polymerized by use of heat or light, whereby a hydrogel orhydrogel-forming polymer may be prepared. The resultant hydrogel orhydrogel-forming polymer is mechanically crushed or pulverized, theunreacted monomer, the remaining polymerization initiator, etc., areremoved therefrom by washing with water, etc., and thereafter theresultant product is dried, thereby to provide a hydrogel-formingpolymer for constituting the vessel or sheet according to the presentinvention.

Further, when the monomer for providing the water-soluble or hydrophilicpolymer is liquid, the multi-functional monomer and polymerizationinitiator are added into the monomer, the monomer is polymerized by bulkpolymerization by use of heat or light, the resultant product ismechanically crushed, the unreacted monomer and the remainingmulti-functional monomer are removed therefrom by extraction with water,etc., and the product is dried, whereby a hydrogel or hydrogel-formingpolymer according to the present invention may be provided.

On the other hand, when the hydrogel or polymer according to the presentinvention in a micro-bead shape is intended to be prepared, it ispossible to use an emulsion polymerization method, a suspensionpolymerization method, a precipitation polymerization method, etc. Inview of the control of the resultant particle size, a reverse-phasesuspension polymerization method may particularly preferably be used. Inthe reverse-phase suspension polymerization method, as a dispersionmedium, an organic solvent (e.g., saturated hydrocarbon such as hexane)which does not dissolve the monomer and the resultant polymer ispreferred. In addition, it is also possible to use a surfactant (e.g., anonionic surfactant such as sorbitan fatty acid ester) as a suspensionauxiliary in combination with the above organic solvent.

The particle size of the resultant micro-bead may be controlled by thekind or amount of the surfactant to be added, the stirring speed, etc.As the polymerization initiator, either of a water-solublepolymerization initiator, and a water-insoluble polymerization initiatormay be used.

When the hydrogel or polymer according to the present invention isformed into a fiber shape, film shape, etc., e.g., it is possible to usea method wherein an aqueous solution of a water-soluble polymer isextruded into an organic solvent which is unmixable with water by usinga die, etc., to form each of the predetermined shapes, and then theresultant product is irradiated with light, electron beam, γ-ray, etc.,so as to impart a crosslinked structure to the polymer. Further, it isalso possible to use a method wherein the above water-soluble polymer isdissolved in an organic solvent or water, is shaped by a solvent castingmethod, and then is irradiated with light, electron beam, γ-ray, etc.,so as to impart a crosslinked structure to the polymer.

In general, the crop cultivation under high-temperature andover-humidity condition is liable to cause a phenomenon such as stemspindly growth, or branching or blooming defectiveness, so as to lowerthe value of the agricultural products. Further, the problem of such avalue decrease can also occur in some cases, depending on the characterof the race of the plant. In such a case, it is preferred to use adwarfing agent having an effect of suppressing the extension of thestem, etc., so as to promote the branching and blooming, as desired. Inthe present invention, in an embodiment using the hydrogel-formingpolymer having a crosslinked structure may preferably be a polymer whichshows a decrease in water absorption magnification along with anincrease in a temperature range of not lower than 0° C. and not higherthan 70° C., and exhibits a water absorption magnification which isreversibly changeable with respect to temperature, when the dwarfingagent is incorporated into the inside of the hydrogel or polymer, theplant-cultivating support, soil-modifying agent, vessel, or sheetcomprising the resultant hydrogel or polymer as a constitution elementthereof discharges therefrom the dwarfing agent to the outside (e.g.,into soil) at a high temperature so as to suppress the stem elongationof the plant. On the other hand, at a lower temperature at which thedemand for the dwarfing agent becomes low, the dwarfing agent is notdischarged from the hydrogel or polymer, and therefore persistence ofthe effect of the dwarfing agent is improved remarkably.

In general, the necessity for a weed killer also becomes greater at ahigh temperature as compared with that at a low temperature.Accordingly, when the weed killer is incorporated into the hydrogel orpolymer according to the present invention, the effect of the weedkiller and the persistence thereof are remarkably improved on the basisof the same storage-discharge mechanism as described above.

Shape and Material of Vessel/sheet

The shape of the plant-growing vessel according to the present inventionis not particularly limited as long as the above-mentioned“hydrogel-forming polymer having a crosslinked structure” is disposedinside thereof, but may be formed into one of known various shapes suchas cotyle-type, pot-type, planter-type, tray-type, etc.

The schematic sectional view of FIG. 1 shows an embodiment (pot-type) ofthe growing vessel according to the present invention. Referring to FIG.1, a layer 2 comprising a “hydrogel-forming polymer having a crosslinkedstructure” is disposed in the inside of a pot-type vessel 1 having abottom 1 a and a side wall portion 1 b. Of course, it is possible thatone or more holes (not shown) may be provided in the bottom 1 a or sidewall portion 1 b as desired.

Similarly, the shape of the plant sheet according to the presentinvention is not particularly limited as long as the above-mentioned“hydrogel-forming polymer having a crosslinked structure” is disposed onthe surface of at least a portion thereof, but may be formed into one ofvarious kinds of known shapes.

The schematic sectional view of FIG. 2 shows an embodiment of thegrowing sheet according to the present invention. Referring to FIG. 2, alayer 2 a comprising a “hydrogel-forming polymer having a crosslinkedstructure” is disposed on one of the surfaces of a sheet base material11 a. On the surface (back) of the sheet base material 11 a disposedopposite to the face on which the polymer layer 2 a is disposed, a layer3 comprising a sticking agent or adhesive (comprising carboxymethylcellulose (CMC), etc.) may be disposed as desired.

Further, as shown in FIG. 3, a sheet 4 having a releasing property maybe disposed on the sticking agent/adhesive layer 3 as desired. When thesheet 11 of such an embodiment as shown in FIG. 3 is used, the sheet 11may easily be placed at a desired location of a conventional vessel (notshown) by tearing off the releasing sheet 4, and thereafter disposingthe sheet 11 in the conventional vessel.

The sheet according to the present invention may be formed into a shapehaving a partition (internal dividing wall) as desired.

The schematic perspective views of FIG. 4A to FIG. 4B show an example ofthe embodiment of the sheet according to the present invention having apartition. FIG. 4A shows an example of the single cell-type partitionform (with an extension portion), and FIG. 4B shows an example of the 4(four) cell-type partition form. The number of the “cell” to be formedby these partitions is not particularly limited, but may preferably beabout 1-10000 (more preferably about 10-1000) in view of efficientutilization or efficiency of the cultivating area. In thesepartition-type sheet 12 according to the present invention, the layer(not shown) comprising the “hydrogel-forming polymer having acrosslinked structure” is disposed on at least a portion of the surface5 of the partition on which a plant is to be disposed.

As shown in the schematic plan view of FIG. 5, when the partition-typesheet 12 according to the present invention is used in combination with“another vessel” 6 (conventional vessel is also usable), the removal ofa plant at the time of the transfer thereof becomes very easy byutilizing the attachment and detachment between the sheet 12 and theother vessel 6. In other words, when the grown plant (not shown) isintended to be removed from the vessel 6 or sheet 12, the removal of theplant becomes extremely easy by pulling out the partition 12 from thevessel 6 in advance. The above-mentioned other vessel 6 may also be aconventional vessel, or a plant-growing vessel (i.e., vessel accordingto the present invention) wherein a layer 2 of the “hydrogel-formingpolymer” is disposed in the inside thereof as desired.

The material for the vessel or sheet according to the present inventionis not particularly limited, but may appropriately be one of knownmaterials such as ceramic or earthenware (unglazed pottery), metal,wood, plastic, and paper.

Embodiment of Polymer Arrangement

In the present invention, the location, area, shape (e.g., either of anintermittent layer or continuous layer), or means of disposing thehydrogel-forming polymer is not particularly limited as long as thepolymer is disposed in the inside of the growing vessel.

The location of the above-mentioned polymer disposed in the vessel mayfor example be either of the bottom face 1 a or the side face 1 b(FIG. 1) of the vessel, but the polymer may preferably be disposed onthe side face 1 b of the vessel in view of easiness in retaining theplant by the swelling of the polymer.

In the present invention, in order to effectively exhibit the functionof the hydrogel-forming polymer, when the area of internal surface ofthe vessel (or the area of one of the side surfaces of a sheet) isdenoted by Sa, and the area on which the hydrogel-forming polymer hasbeen disposed is denoted by Sp, the ratio (Sp/Sa)×100 of these areas maypreferably be about 10% or more, more preferably about 50% or more(particularly about 70% or more).

In the present invention, the layer 2 or 2 a of the hydrogel-formingpolymer may be a continuous layer or an intermittent layer. Such anintermittent layer may easily be formed by an arbitrary measure such asscreen printing. When the intermittent layer is intended to be formed,the plan shape thereof may be an arbitrary shape such as checkeredpattern-type as shown in FIG. 6A, and spot-type as shown in FIG. 6B.

When the layer 2 or 2 a of the hydrogel-forming polymer is disposed onthe base material 1 of the vessel or sheet, the embodiment of thearrangement is not particularly limited. In view of easiness in thearrangement thereof, there may preferably be used any of an embodimentwherein the polymer layer 2 is disposed directly on the base material 1(FIG. 7A), an embodiment wherein the polymer layer 2 is disposed on alayer 7 of a sticking agent or adhesive which is disposed on the basematerial 1 (FIG. 7B), or an embodiment wherein the polymer layer 2 inthe shape of an arbitrary form such as particulate-type andindeterminate-type is disposed on a layer 7 of a sticking agent oradhesive which is disposed on the base material 1 (FIG. 7C). In theabove-mentioned embodiment of FIG. 7A, in order to impart an adhesiveproperty to the polymer layer 2 with respect to the base material 1 orto enhance the adhesive property, it is possible that a hydrogel-formingpolymer is mixed or dispersed in the sticking agent or adhesive, andthen is formed into the above-mentioned polymer layer 2 as desired. Insuch a case, it is preferred to use the sticking agent or adhesive in anamount about 0.01-10 wt. parts (more preferably, about 0.1-2 wt. parts)with respect to 10 wt. parts of the hydrogel-forming polymer.

As the above “sticking agent or adhesive”, a known sticking agent oradhesive may be used without particular limitation, but it is preferredto use a substance which is substantially non-toxic or has a lowtoxicity to a plant to be cultivated, as the above-mentioned substance.Specific examples of such a sticking agent or adhesive may include:rubber or latex-type (natural rubber-type, isoprene latex-type), acrylicresin-type (acrylic-type, cyano-acrylate-type), epoxy resin-type,urethane resin-type, protein-type (soybean protein-type, gluten-type),starch-type (starch-type, dextrin-type), and cellulose-type (CMC-type,nitro-cellulose-type).

In any of the above-mentioned embodiments of the vessel or sheet, inorder to effectively exhibit the function of the hydrogel-formingpolymer, when the area of internal surface of the vessel (or the area ofone of the side surfaces of a sheet) is denoted by Sa, and the weight ofthe disposed hydrogel-forming polymer is denoted by Mp, the amount ofthe application of the polymer (Mp/Sa) may preferably be about 0.0001g/cm² (0.1 mg/cm²) or more, more preferably about 0.001 g/cm² (1mg/cm²)to 0.2 g/cm² (particularly about 0.002 g/cm² (2 mg/cm²) to 0.1 g/cm²).

Process for Producing Plant-growing Vessel or Sheet

The process for producing a shaped product (vessel or sheet), the basematerial surface of which the hydrogel has been fixed is notparticularly limited, but, e.g., either of the following two processesmay preferably be used.

The first process is one wherein the material to be used as the basematerial is shaped into a vessel or sheet such as pot and planter inadvance, then a substance (such as sticking agent and adhesive) having afunction of fixing the hydrogel-forming polymer or hydrogel is appliedonto a face for forming the internal surface of the shaped product, andthe hydrogel-forming polymer or hydrogel is fixed onto the thus appliedsubstance.

The second process is one wherein a substance (such as sticking agentand adhesive) having a function of fixing the hydrogel-forming polymeror hydrogel is applied onto a surface of a sheet or film to be formedinto the base material, the hydrogel-forming polymer or hydrogel isfixed onto the thus applied substance, and then the resultant product isshaped into a form such as pot or planter by a pressure molding process,etc.

When the above-mentioned first process is used, the material to beformed into a base material may be shaped into a form such as pot orplanter by various kinds of molding processes such as injection molding,pressure molding, and blow molding. As the above substance for fixingthe hydrogel-forming polymer or hydrogel to the internal surface of theshaped product, a known substance such as sticking agent or adhesivewhich is ordinarily commercially available may be used withoutparticular limitation, but it is preferred to use a substance which issubstantially non-toxic or has a low toxicity to a plant, as theabove-mentioned substance. Specific examples of such a sticking agent oradhesive may include: sticking agents and adhesives of rubber-type,latex-type, acrylic resin-type, epoxy resin-type, urethane resin-type,protein-type, starch-type, and cellulose-type.

It is possible that the above adhesive or sticking agent is applied ontothe internal surface of the above-mentioned shaped product by spraying,casting, or dipping, etc., and the hydrogel-forming polymer or hydrogelis fixed onto the thus applied adhesive or sticking agent. Further, inplace of the above-mentioned adhesive, sticking agent, etc., it is alsopossible that a double-side adhesive-coated tape onto which theabove-mentioned sticking agent, etc., has been applied in advance, isattached to the internal surface of the above-mentioned shaped product,and the hydrogel-forming polymer or a hydrogel is fixed onto the tape.

In the above first process, it is also possible that the material to beformed into the base material is shaped into a form such as pot andplanter by injection molding, etc., a material obtained by dispersing ahydrogel-forming polymer or hydrogel in a thermoplastic elastomer, etc.,is applied to the internal surface of the resultant shaped product byinjection molding using a two-color molding process, whereby thehydrogel-forming polymer or hydrogel may be fixed onto the internalsurface of the shaped product of the base material.

On the another hand, in the second process, it is possible that asubstance (such as above-mentioned adhesive and sticking agent) capableof fixing the hydrogel-forming polymer or hydrogel is applied onto thesurface of sheet or film to be formed into the base material byspraying, casting, etc., or the above-mentioned double-sideadhesive-coated tape is attached thereonto, and then thehydrogel-forming polymer or hydrogel is fixed onto the thus applied orattached substance, and the resultant base material is shaped bypressure molding, etc. Further, a material obtained by dispersing thehydrogel-forming polymer or hydrogel in a thermoplastic elastomer, etc.,is shaped into a multi-layer sheet or multi-layer film by a multi-layerextrusion process together with a material to be formed into the basematerial so that the hydrogel-forming polymer or hydrogel is fixed ontothe base material sheet or base material film, and then the resultantbase material is shaped by pressure molding, etc.

Method of Using Plant-growing Vessel or Sheet

As the method of effectively transferring (or plant-embedding) a plantby using the vessel or sheet having the hydrogel-forming polymerdisposed therein according to the present invention, e.g., the followingmethods of using the vessel or sheet may preferably be used.

(1) There is used a vessel or a sheet shaped into a vessel-type formwhich contains hydrogel-forming polymer particles disposed therein in anamount such that the inside of the vessel is filled with the resultanthydrogel when the polymer particles absorb water. Then, at least aportion of a plant is placed in the vessel or sheet, and thereafter a(fertilizer) solution, etc., is added into the vessel so as to swell thehydrogel-forming polymer particles, thereby to fix the plant.

(2) There is used a vessel or a sheet shaped into a vessel-type formwhich contains hydrogel-forming polymer particles disposed therein in anamount such that the inside of the vessel is filled with the resultanthydrogel when the polymer particles absorb water. Then, a solution,etc., is added into the vessel or sheet so as to fill the vessel orsheet with the resultant hydrogel, and thereafter at least a portion ofa plant is inserted into the gel, thereby to fix the plant.

When the above-mentioned method (1) or (2) is used, since the swollenhydrogel particles containing water have an appropriate fluidity, theplant may smoothly be transferred without damaging the plant. Further,in the case of a minute tissue such as seed, adventive embryo to beprovided by tissue culture, and PLB (Protocorm Like Body; a tissueprovided by tissue culture, which is similar to spherical tissue formedby the germination of a seed), it is also possible to use a method ofsimply placing the tissue, etc., on the hydrogel.

(3) There is used a vessel or a sheet shaped into a vessel-type formwhich contains hydrogel-forming polymer particles disposed therein in anamount such that the inside of the vessel is not sufficiently filledwith the resultant hydrogel when the polymer particles absorb water. Atleast a portion of a plant is placed in the vessel together with a plantsupporting support, and then a solution, etc., is added into the vesselso as to swell the hydrogel-forming polymer, thereby to fix the plant.

(4) A plant is wrapped in a sheet (sheet according to the presentinvention) which has been coated with particles of the hydrogel-formingpolymer, and is planted or embedded into an usual vessel or support, andthen a solution, etc., is added into the vessel so as to swell thehydrogel-forming polymer, thereby to fix the plant.

When any of the above-mentioned (1) to (4) is used, the plant may easilybe attached or fixed to the support immediately.

Transferring Method

On the another hand, as the method of effectively transferring a plant(or taking out a plant) by using the vessel or sheet having thehydrogel-forming polymer disposed therein according to the presentinvention, e.g., the following methods of using the vessel or sheet maypreferably be used.

(1) A method wherein a large excess of water is supplied to the vesselor sheet so as to enhance the fluidity of the hydrogel, thereby to takeout the plant without damaging the plant.

(2) A method of using a vessel or sheet having the hydrogel-formingpolymer comprising a polymer having a carboxyl group, wherein thehydrogel in a swollen state is shrunk by adding thereto a highconcentration of calcium solution or calcium salt powder, thereby totake out the plant without damaging the plant.

(3) A method of using a vessel or sheet having the hydrogel-formingpolymer having a property such that the water absorption magnificationis decreased along with an increase in temperature in the temperaturerange of not lower than 0° C. and not higher than 70° C., and the changein the water absorption magnification is reversible with respect totemperature, wherein the vessel or sheet is warmed up to a temperaturewhich does not adversely affect a plant so that the swollen hydrogelparticles are caused to discharge the water content contained therein tobe shrunk, whereby the plant is taken out without damaging the plant.

(4) A method of using a vessel or sheet having the hydrogel-formingpolymer having a property such that the water absorption magnificationis decreased along with an increase in temperature in the temperaturerange of not lower than 0° C. and not higher than 70° C., and the changein the water absorption magnification is reversible with respect totemperature, wherein the vessel or sheet is supplied with warm waterwhich does not adversely affect a plant, so that the swollen hydrogelparticles are caused to discharge the water content contained therein tobe shrunk, and the fluidity of the gel particles is enhanced, wherebythe plant is taken out without damaging the plant. The temperature ofthe above warm water may preferably be about 45° C. or less (morepreferably about 40° C. or less), while the temperature may somewhatvary depending on the kind of the plant.

When any of the above-mentioned method (1) to (4) is used, the plant mayeasily be taken out from the vessel immediately without damaging theplant.

Method of Removing Liquid Substance Such as Water

In view of an improvement in workability, reduction in transportingcosts, etc., at the time of the transportation (such as shipment), it isimportant to reduce the weight of the cultivating vessel. Further, atthe time of the transportation, the plant is put under a closed-typeenvironment (e.g., a state wherein the plant is packed with cellophanetogether with a vessel, and put in a corrugated board) in many cases.Under such a condition, in order to prevent the damage to the plant evenin a wetted state, it is important to reduce the amount of watercontained in the cultivating vessel to as small amount as possible.

In a case of using the vessel or sheet according to the presentinvention which has the hydrogel-forming polymer disposed therein, whenthe water content or liquid such as fertilizer solution in the vessel orsheet becomes unnecessary, for example, the liquid may preferably beremoved by the following method.

(1) A method wherein the hydrogel particles are dried so that thehydrogel particles are caused to discharge water contained therein, andthe weight thereof is reduced. However, it is necessary to conduct sucha method in a certain range such that the resultant “concentration ofnutrient” does not substantially affect the plant adversely.

(2) A method of using a vessel or sheet having the hydrogel-formingpolymer comprising a polymer having a carboxyl group, wherein thehydrogel in a swollen state is shrunk by adding thereto a highconcentration of calcium solution or calcium salt powder, thereby tocause the hydrogel to discharge a liquid such as water content andfertilizer solution.

(3) A method of using a vessel or sheet having the hydrogel-formingpolymer having a property such that the water absorption magnificationis decreased along with an increase in temperature in the temperaturerange of not lower than 0° C. and not higher than 70° C., and the changein the water absorption magnification is reversible with respect totemperature, wherein the vessel or sheet is warmed up to a temperaturewhich does not adversely affect a plant so that the swollen hydrogelparticles are caused to discharge a liquid such as water content andfertilizer solution which has been contained in the hydrogel particles.

In the prior art, the water which has been supplied to a plant beforethe shipment thereof may cause a problem such that it weaken theresistance to dryness so as to decrease the persistence of the flower,and it decrease the sugar content in the resultant fruit. Also in orderto solve such a problem, it is preferred to remove water content, etc.,by using the above-mentioned (1) to (3) (preferably, by the method (2)or (3)) in advance, before the shipment.

EXAMPLES

Hereinbelow, the present invention will be described in more detail withreference to Examples.

Example 1 Preparation of Water-Retaining Support

10 g (140 mmol) of acrylic acid and 0.05 g (0.32 mmol) ofN,N′-methylenebis acrylamide were dissolved in 26 ml of distilled water.Into thus obtained solution, 0.52 g (7 mmol) of calcium hydroxide and 14ml (14 mmol) of 1N-aqueous potassium hydroxide solution were added.While the resultant mixture was stirred at room temperature under astream of nitrogen, 0.02 g of ammonium persulfate and 0.01 g of ascorbicacid were added thereto. After 5 minutes counted from the addition ofthe ammonium persulfate and ascorbic acid, the temperature of thereaction mixture was abruptly increased so that the mixture wasconverted into a gel. Further, the reaction was continued as it was for1 hour under the stream of nitrogen.

200 ml of ethyl alcohol was added to the resultant product, and waspulverized in a mixer. The resultant gel was separated from thepulverized product and was subjected to vacuum drying.

A predetermined amount (0.2 g) of thus obtained hydrogel-forming polymer(water-retaining support according to the present invention) was weighedin a platinum crucible, was subjected to ashing in an electric furnace(at 700° C.), and was then dissolved in 5 ml of 1N-hydrochloric acid.Distilled water was added to the resultant product to provide a totalvolume of 50 ml. When the potassium ion content therein was determinedby means of an atomic absorption spectrophotometer (mfd. by SeikoElectronics K. K.; trade name: SAS-760), it was found to be 1.3 mmol/g.

The calcium ion absorption (amount) of the above water-retaining supportand its water absorption magnification in ion-exchange water (electricconductivity: 2.5 μS/cm) were 19 mg/g and 377 (times), respectively.

Example 2 Preparation of Water-Retaining Support

5 g of a commercially available sodium polyacrylate-type highlywater-absorbing resin (trade name: Acryhope; mfd. by Nippon Shokubai K.K.) was swollen with 1 L of ion-exchange water. To the thus swollenhighly water-absorbing resin, an aqueous solution which had beenobtained by dissolving 2.9 g of calcium chloride (dihydrate salt) in 500ml of ion-exchange water was added. As the resultant mixture was leftstanding for 1 hour at room temperature (25° C.) while beingoccasionally stirred, the sodium salt of carboxyl group was partiallysubstituted by the calcium salt.

The resultant supernatant above the swollen resin was discarded, 2 L ofion-exchange water was added to the resultant gel so as to wash the gel,and then the supernatant above the swollen resin was discarded again.After the operation of washing the gel with ion-exchange water wasrepeated five times, 1 L of ethyl alcohol was added to the gel to shrinkthe gel, and the gel was separated from the resultant mixture and wassubjected to vacuum drying.

A predetermined amount of thus obtained water-retaining support wasweighed in a platinum crucible and was subjected to ashing in anelectric furnace, dissolved in hydrochloric acid, and total volumethereof was adjusted to the fixed value, and the sodium ion contenttherein was determined by atomic absorption spectrometry, in the samemanner as in Example 1. As a result, the sodium ion content was found tobe 2.2 mmol/g. Further, the calcium ion content was 2.1 mmol/g.

The calcium ion absorption of the above water-retaining support and itswater absorption magnification in ion-exchange water (electricconductivity: 2.5 US/cm) were 36 mg/g and 175 (times), respectively.

Example 3 Preparation of Water-Retaining Support

20 g of a commercially available sodium polyacrylate highlywater-absorbing resin (trade name: Acryhope; mfd. by Nippon Shokubai K.K.) was swollen with 1 L of ion-exchange water. To the thus swollenresin, 170 ml of 1N-hydrochloric acid was added. While the mixture wasoccasionally stirred at room temperature (25° C.), the sodium salt ofcarboxyl group was substituted by carboxylic acid for 1 hour.

The resultant supernatant above the swollen resin was discarded, 2 L ofion-exchange water was added to the resultant gel so as to wash the gel,and then the supernatant above the swollen resin was discarded again.Further, 1 L of ion-exchange water and 20 ml of IN-hydrochloric acidwere added to the resultant gel. After the thus obtained mixture wasleft standing for 1 hour at room temperature (25° C.) while the mixturewas occasionally stirred, the gel was separated therefrom and wassubjected to vacuum drying.

A predetermined amount of the thus obtained polyacrylic acid crosslinkedproduct was weighed in a platinum crucible and, in the same manner as inExample 1, was subjected to ashing in an electric furnace, dissolved inhydrochloric acid, and the total volume thereof was adjusted to thefixed value, and the alkali metal ion content therein was determined byatomic absorption spectrometry. As a result, the alkali metal ioncontent was found to be 0.01 mmol/g or less, and the water absorptionmagnification of the polymer in ion-exchange water (electricconductivity: 2.5 μS/cm) was 14 (times).

2 g of the above-mentioned polyacrylic acid crosslinked product wasswollen with 500 ml of ion-exchange water. 2.78 ml of 1N-aqueouspotassium hydroxide solution was added to the thus swollen product, andwhile the mixture was occasionally stirred at room temperature (25° C.),and its carboxylic acid was partially substituted by potassium salt for1 hour. The resultant supernatant was discarded, and the gel wasseparated from the mixture and was subjected to vacuum drying. Apredetermined amount of the resultant water-retaining support accordingto the present invention was weighed in a platinum crucible, subjectedto ashing in an electric furnace, dissolved in hydrochloric acid, andthe total volume thereof was adjusted to the fixed value, and thepotassium ion content therein was determined by atomic absorptionspectrometry. As a result, the potassium ion content was found to be 1.3mmol/g.

The calcium ion absorption of the above water-retaining support and itswater absorption magnification in ion-exchange water (electricconductivity: 2.5 μS/cm) were 21 mg/g and 171 (times), respectively.

Example 4 Preparation of Water-Retaining Support

A water-retaining support according to the present invention wasobtained in the same manner as in Example 3 except that the amount ofthe 1N-aqueous potassium hydroxide solution to be used for the potassiumsalt substitution was changed to 5.56 ml.

A predetermined amount of the thus obtained water-retaining support wasweighed in a platinum crucible and, in the same manner as in Example 1,was subjected to ashing in an electric furnace, dissolved inhydrochloric acid, and the total volume thereof was adjusted to thefixed value, and the potassium ion content therein was determined byatomic absorption spectrometry. As a result, the potassium ion contentwas found to be 2.5 mmol/g.

The calcium ion absorption of the above water-retaining support and itswater absorption magnification in ion-exchange water (electricconductivity: 2.5 μS/cm) were 40 mg/g and 185 (times), respectively.

Example 5 Preparation of Thermo-sensitive Water-retaining Support

15 g of N-isopropyl acrylamide (NIPAAm, mfd. by Kojin K. K.), 0.47 g ofacrylic acid, 0.1 g of N,N′-methylenebis-acrylamide (Bis), 0.2 g ofammonium persulfate, 6.6 mL of 1N-NaOH, and 0.1 mL ofN,N,N′-N′-tetramethylethylene diamine was dissolved in 90 mL ofdistilled water. The resultant mixture was subjected to polymerizationfor 4 hours at room temperature, thereby to obtain a poly-N-isopropylacrylamide (PNIPAAm) hydrogel having a crosslinked structure.

The resultant gel was mechanically crushed by means of a mixer, and theresultant product was dispersed in one liter of distilled water andcooled to 4° C. Thereafter, the resultant mixture was warmed to 50° C.so as to be shrunk, and the resultant supernatant liquid was discarded.Such a washing operation was repeated twice, thereby to remove theunreacted monomer and the remaining polymerization initiator. Further,the product was dried under vacuum (100° C., 24 hours), thereby toobtain a water-retaining support according to the present invention. Inthe thus obtained support, the water absorption magnification wasdecreased along with an increase in temperature, and the change in thewater absorption magnification was reversible with respect totemperature.

The calcium ion absorption of the above water-retaining support and itswater absorption magnification in ion-exchange water (electricconductivity: 2.5 μS/cm) were 9 mg/g and 167 (times), respectively.

The water absorption magnification of the thus obtained water-retainingsupport with respect to a commercially available powder horticulturalfertilizer (trade name: Hyponex 20-20-20, mfd. by Hyponex Japan K. K.; 1g/L) was measured at 19° C. and 26° C. according to the method asdescribed hereinabove. The thus measured water absorption magnificationwas about 72 at 19° C., and about 52 at 26° C.

Comparative Example 1 Comparative Example for Example 3

A water-retaining support of Comparative Example was obtained in thesame manner as in Example 3 except that the amount of the 1N-aqueouspotassium hydroxide solution was changed to 0.35 ml. A predeterminedamount of the thus obtained water-retaining support was weighed in aplatinum crucible, and in the same manner as in Example 1, was subjectedto ashing in an electric furnace, dissolved in hydrochloric acid, andthe total volume thereof was adjusted to the fixed value, and thepotassium ion content therein was determined by atomic absorptionspectrometry. As a result, the potassium ion content was found to be0.15 mmol/g.

The calcium ion absorption of the above water-retaining support and itswater absorption magnification in ion-exchange water (electricconductivity: 2.5 μS/cm) were 2 mg/g and 75, respectively.

Comparative Example 2 Comparative Example for Example 3

A water-retaining support of Comparative Example was obtained in thesame manner as in Example 3 except that the amount of the 1N-aqueouspotassium hydroxide solution was changed to 8.34 ml. A predeterminedamount of the thus obtained water-retaining support was weighed in aplatinum crucible, and in the same manner as in Example 1, was subjectedto ashing in an electric furnace, dissolved in hydrochloric acid, andthe total volume thereof was adjusted to the fixed value, and thepotassium ion content therein was determined by atomic absorptionspectrometry. As a result, the potassium ion content was found to be tobe 3.6 mmol/g.

The calcium ion absorption of the above water-retaining support and itswater absorption magnification in ion-exchange water (electricconductivity: 2.5 μS/cm) were 55 mg/g and 191, respectively.

Comparative Example 3 Examples of Commercially Available Resins

With respect to three kinds of commercially available highlywater-absorbing resins (trade name: Acryhope, mfd. by Nippon Shokubai K.K.; trade name: Diawet, mfd. by Mitsubishi Chemical K. K.; and tradename: Sumicagel, mfd. by Sumitomo Chemical K. K.), the calcium ionabsorption and the water absorption magnification in ion-exchange water(electric conductivity: 2.5 μS/cm) were measured. The thus obtainedresults are shown in the following Table 1 together with the resultsobtained in Examples 1 to 5 and Comparative Examples 1 and 2.

Example 6 Test of Seed Germination

Synthetic water (as shown in Table 2 appearing herein below) having acomposition similar to that of underground water in Kumano district ofEnzan City in Yamanashi Prefecture was prepared. Into a test tube(having a diameter of 2.5 cm and a height of 15 cm), 16 ml of the abovesynthetic water and 160 mg (1 wt. %) of each of the water-retainingsupports of the present invention prepared in Examples 1, 2, 3, and 4was introduced. After the resultant mixture was sufficiently stirred,the mixture was left standing for 30 minutes at 25° C., thereby toprepare a gel-like culture medium comprising the water-retaining supportwhich had absorbed the synthetic water.

Seeds of white radish sprouts (Takii Shubyo K. K.) were uniformly put oneach of the surfaces of the thus obtained gel culture medium in the testtubes in an mount of 5 seeds/test tube, and the test tube was cappedwith a silicone plug having a 6-mm diameter hole filled with cotton.

The thus capped test tube was cultured for 4 days in a culture chamber(25° C., illumination intensity of 2000 lux, 16h-daytime (fluorescentlight illumination)), and the ratio of germination (number of germinatedseeds/5 (seeds)×100(%)) was investigated.

In the above-mentioned germination and germination activity test, thecase wherein the seed coat was torn and the cotyledon was unfolded wasdefined as the occurrence of germination, and the other cases aredefined as no occurrence of germination. The length of the shoot portionwas measured as the average stem length from the base portion to theleaf tip of the germinated seed, while the length of the root portionwas measured as the average root length from the base portion to the tipof the main root of the germinated seed. Further, the appearance of theroot tip, etc., was observed.

The thus obtained results are inclusively shown in Table 3. In thewater-retaining support according to the present invention prepared inExamples 1, 2, 3, and 4, germination was 100% in all the groups, and thegrowth of white radish was very good in both shoots and roots.

Comparative Example 4 Comparative Example for Example 6

The germination tests were conducted in the same manner as in Example 6with respect to the two kinds of water-retaining supports prepared inComparative Examples 1 and 2, and three kinds of commercially availablehighly water-absorbing resins (Acryhope, Diawet, and Sumicagel) used inComparative Example 3.

In the case wherein the water-retaining support of Comparative Example 1was used, the water absorption magnification was so insufficient thatthe culture medium was in the form of a liquid, whereby the seeds weresunk in the culture medium and showed no germination thereof. In thecases where the water-retaining supports of Comparative Example 2 andthe commercially available highly water-absorbing resins were used, theseeds showed 100% germination, but the tip of the root caused browningand fatal withering after the root origination thereof, and the growthof the shoot portion was completely suppressed (as shown in thefollowing Table 3).

TABLE 1 Calcium Absorption and Water absorption Magnification ofWater-Retaining Support Calcium ion absorption Water absorption Sample(mg/g) magnification Example 1 19 377 Example 2 36 175 Example 3 21 171Example 4 40 185 Example 5 9 167 Comp.Ex.1 2 75 Comp.Ex.2 55 191Acryhope 150 196 Diawet 140 172 Sumicagel 100 326

TABLE 1 Calcium Absorption and Water absorption Magnification ofWater-Retaining Support Calcium ion absorption Water absorption Sample(mg/g) magnification Example 1 19 377 Example 2 36 175 Example 3 21 171Example 4 40 185 Example 5 9 167 Comp.Ex.1 2 75 Comp.Ex.2 55 191Acryhope 150 196 Diawet 140 172 Sumicagel 100 326

(Respective components were dissolved in ion-exchange water at itspredetermined concentration, and then pH of the resultant mixture wereadjusted to 7 by using hydrochloric acid.)

TABLE 3 Results of Germination Rate and Growth Test for White RadishGermination Shoot length Root length Comments on Sample rate (%) (cm)(cm) appearance Example 1 100 6.5 4.2 Good Example 2 100 4.5 2.5 GoodExample 3 100 5.5 3.1 Good Example 4 100 5.5 3.1 Good Example 5 100 7.04.3 Good Comp.Ex.1 0 0 0 Seeds sunk Comp.Ex.2 100 2.0 0 Root tip causedAcryhope 100 1.0 0 browning and Diawet 100 1.0 0 fatal witheringSumicagel 100 1.0 0

Example 7 Surface-Crosslinked Water-Retaining Support

Into a mixer, 100 g of a hydrogel-forming polymer (in a powder form)obtained in the same manner as in Example 1 were introduced. While thepolymer was being stirred, 4 g of an aqueous crosslinking agent solutionwhich had been obtained by dissolving 10 wt. % of ethylene glycoldiglycidyl ether in 15 wt. % of aqueous sodium propinate solution wasadded to the polymer and was sufficiently mixed therewith. The resultantmixture was heat-treated at 150° C. for about 20 minutes, thereby toobtain a surface-crosslinked water-retaining support for plant accordingto the present invention.

The potassium ion content of the thus obtained water-retaining supportwas measured in the same manner as in Example 1, and the potassium ioncontent was found to be 1.2 mmol/g.

The calcium ion absorption of the above water-retaining support and itswater absorption magnification in ion-exchange water (electricconductivity: 2.5 μS/cm) was 16 mg/g and 314 (times), respectively.

3 g of the above water-retaining support was introduced into a plant box(mfd. by Shibata Hario K. K., comprising polycarbonate, upperportion=75×75 mm, lower portion=65×65 mm, height=100 mm). When thesupport was caused to absorb 150 ml of a Hyponex solution (Hyponex 7-6-9(mfd. by Hyponex Japan K. K.); 1 g/L), the solution was rapidly absorbedthereinto, and the support was entirely solidified in a state whereinappropriate voids were retained among the swollen water-retainingsupport particles. To the above gel culture medium, orchid (cymbidium)plantlets of MFMM (Cym. MELODY FAIR ‘Marilyn Monroe’) were transplanted.After the plantlets were cultivated for 60 days in a greenhouse, it wasobserved that all of the flower, stem, and root portions of the orchidswere well grown.

Example 8 pH Measurement of Water-Retaining Support

Into 100 ml of ion-exchange water, 1 g of each kind of syntheticpolymers in a dry state as shown in the following Table4 was dispersed.After 1 hour counted from the mixing, the pH value of the resultantmixture was measured by use of a pH meter (mfd. by Yokogawa Electric K.K.; trade name: PH-81). It was confirmed that the water-retainingsupports of the present invention obtained in Examples 1 to 5 wereweakly acidic (pH 4.7 to 6.0) which were suitable for plant growth.

TABLE 4 Sample pH Example 1 4.8 Example 2 6.0 Example 3 4.7 Example 45.0 Example 5 5.4 Comp.Ex.1 3.7 Comp.Ex.2 5.5 Acryhope 7.0 Diawet 7.0Sumicagel 7.9

Example 9 Culture Method Using Water-Retaining Support

In a test tube (having a diameter of 2.5 cm and a height of 15 cm), 16ml of a culture liquid (containing 20 g/L of sucrose and 100 g/L ofbanana) including a commercially available powder type horticulturalfertilizer (trade name: Hyponex 7-6-19, mfd. by Hyponex Japan K. K., 3.5g/L) was mixed with and dispersed into 400 mg of the driedwater-retaining support prepared in Example 3. After the mixture wassterilized by an autoclave (121° C., 1.2 kg/cm², 20 minutes), themixture was left standing at room temperature, thereby to prepare ahydrogel culture medium.

Into the above-mentioned culture medium in each of test tubes, twoorchid plantlets of YT57 (Cym. LOVELY ANGEL ‘The Two Virgins’) which hadbeen grown so as to have a length of about 1.5 cm were transplanted; andthe plantlets were aseptically cultured for 50 days in a culture chamber(25° C., 3000 Lux, 16h-daytime). The maximum leaf length of eachplantlet was measured, it was found to be 6.7 cm on average. The rootsof plantlets were well elongated. The plantlets were also grown wellafter they were moved into cultivation under greenhouse condition, andexhibited substantially no dying of the leaf tip.

Comparative Example 5 Culture Method Using Agar

YT57 plantlets were cultured for 50 days in the same manner as in theabove-mentioned Example 9 except that 100 mg of agar was added insteadof the dried water-retaining support used in Example 9. The maximum leaflength of each plantlet was measured and it was found to be 6.7 cm onaverage, which was substantially the same as that in the above-mentionedExample. Their roots were well grown in the appearance thereof, butsomewhat dying of the leaf tip was observed after they were moved intocultivation under greenhouse condition. It was presumed that the abovephenomenon was attributable to the fact that the plantlets during theculture were not appropriately acclimated to water stress.

Comparative Example 6 Culture Method Using Commercially Available Resin

YT57 plantlets were cultured for 50 days in the same manner as in theabove-mentioned Example 9 except that 400 mg of Acryhope was addedtherein instead of the dried water-retaining support used in Example 9.No growth was observed in any of the shoot and root portions thereof.

Example 10 Culture Method Using Water-Retaining Support

In a plant box (mfd. by Shibata Hario K. K., comprising polycarbonate,upper portion=75×75 mm, lower portion=65×65 mm, height=100 mm), 1.5 g ofthe dried water-retaining support obtained in Example 5 and 105 ml of aHyponex solution (Hyponex 7-6-9; 2.0 g/L) was mixed and dispersedtogether. After the mixture was sterilized by an autoclave (121° C., 1.2kg/cm², 20 minutes), the mixture was aseptically mixed with 80 ml ofpearlite (mfd. by Nihon Cement K. K.; trade name: Asano-Pearlite No. 3)which had been separately sterilized, thereby to prepare a hydrogelculture medium.

To the above culture medium, orchid plantlets of MFMM (Cym. MELODY FAIR‘Marilyn Monroe’) which had been grown so as to have a length of about 4cm were transplanted in an amount of 16 plants in each box; and theplantlets were aseptically cultured for 50 days in a culture chamber(25° C., 3000 Lux, 16h-daytime). The plantlets were well grown, thestate of their root was very good, and white thick roots, which weresimilar to those obtained in the growth in farm cultivation, wereelongated.

Comparative Example 7 Culture Method Using Agar

MFMM plantlets were cultured for 50 days in the same manner as in theabove-mentioned Example 10 except that agar gel (700 mg) was used aloneinstead of the dried water-retaining support used in Example 10. Theshoot portions were well grown, but the roots were not elongated so muchand the roots were thin which had a form different from those to beelongated in farm cultivation.

Comparative Example 8 Culture Method Using Commercially Available Resin

YT57 plantlets were cultured for 50 days in the same manner as in theabove-mentioned Example 10 except that 1.5 g of Acryhope was addedinstead of the dried water-retaining support used in Example 10. Nogrowth was observed in any of the shoot and root portions.

Example 11 Acclimation During Culture by Water-Retaining Support

Into 20 g of the dried water-retaining support prepared in Example 1,each of amounts of 1000, 800, 600, 400, and 200 cc of a Hyponex solution(Hyponex 7-6-19, 2 g/L, dissolved in synthetic water) was completelyabsorbed so as to form a gel. The pF values of the thus obtained gelswere measured by a pF meter (manufacture by Daiki Rika Kogyo K. K.;DIK-8340) to be 0, 0, 1.8, 2.1, and 2.3, respectively. The water contentin a culture medium immediately after plantlet transplantation in usualculture is decreased by 40 to 80% until the culture-terminating stagedue to the evaporation toward the outside of the vessel and theabsorption thereof by a plant during the culture. In this Example,however, it was found that the pF at termination of the culture changedto the range of 1.8 to 2.3 when the hydrogel-forming polymer was used inthis Example. That is, it is presumed that, in the plantlet cultureusing the water-retaining support according to the present invention, anappropriate water stress is applied to the root of a plant during theculture, thereby to well acclimate the plant.

Comparative Example 9 Water Stress Deficiency in Agar Culture Method

With 1000 cc of the Hyponex solution used in Example 10, 7 g of agar washeated and dissolved. After the mixture was converted into a gel at roomtemperature, the pF value thereof was measured and it was found to be 0(zero). After the gel was dried in a culture chamber, and the pF valueat each of the gel weights of 809 g, 609 g, 409 g, and 209 g wasmeasured. As a result, all of them were found to be 0. Though 40 to 80%of water in a culture medium is usually decreased during culture, it wasfound that the pF value in the agar gel hardly changed. That is, it ispresumed that, in the plantlet culture using the agar gel, no stress isapplied to the root of a plant during the culture at all, wherebypreferable acclimation would not proceed.

TABLE 5 Shifting of pF Value upon Decrease in Water during Culture Wateramount 1000 800 600 400 200 Example 11 0 0 1.8 2.1 2.3 Comparative 0 0 00 0 Example(Agar)

Example 12 Example of Liquid Culture

Into an Erlenmeyer flask (mfd. by Shibata Hario Glass K. K.; volume: 500ml), 200 ml of 1/2 Murashige & Skoog culture medium (containing 20 g/Lof sucrose) was introduced. Then, the dried water-retaining supportprepared in Example 5 was added to the medium at various concentrations(no addition, 0.4 g and 1.0 g), and mixed and dispersed therein. Afterthe mixture was sterilized by an autoclave (121° C., 1.2 kg/cm², 20minutes), the mixture was left standing at room temperature, thereby toprepare a suspension culture medium. The volume ratio of the suspensionculture medium to the gel was about 9:1 in the 0.4 g-addition group, andabout 3:1 in the 1.0 g-addition group.

Into the above-mentioned culture medium, PLB (Protocorm Like Body;systematic cell agglomeration peculiar to an orchid) was transplanted inan amount of 2.0 g in each flask, and aseptically cultured for 22 daysin a culture chamber (25° C., 3000 Lux, 16h-daytime) while the culturemedium was shaken and horizontally rotated (80 revolutions per 1 minutewith a radius of gyration of 27 mm). Thereafter, the resultant freshweight of the PLB was measured, and the state of the PLB and the stateof elution of a browning material into the culture liquid were observed.

As shown in Table 6, it was found that the addition of thewater-retaining support according to the present invention to thesuspension culture system accelerated the propagation of PLB andsuppressed the elution of the browning materials.

TABLE 6 Effect of Addition of Water-Retaining support to Liquid CultureMedium on PLB Propagation in MFMM Water-retaining carrier MultiplicationState of browning concentration (%) rate (times) Form of PLB elution 03.2 Small grain Elution was noticeable 0.2 5.8 Large grain Elution wasmedium 0.5 5.9 Large grain Elution was little

Example 13 Cultivating Method Using Water-Retaining Support

Into 115 ml of the synthetic water shown in Table 2, 100 mg of Hyponexpowder (Hyponex 20-20-20, mfd. by Hyponex Japan K. K.) was dissolved.The resultant solution was completely absorbed in 1 g of each of variouskinds of hydrogel-forming polymer powder, so as to form a gel. Into eachof the above gels, 50 cc of pearlite was added and uniformly mixedtherewith. Each cell of a cell tray (mfd. by Tokan Kosan K. K.; singlecell dimension: 2.5 cm(length)×2.5 cm (width)×4.5 cm (height); cellnumber 10×20=200 holes; with a closed lower portion and an open upperportion) was filled with the thus obtained support. Each of plantlets ofone genus of orchid plant family, Phalaenopsis (Dtps. HappyValentine×Show Girl ‘Mai’), and cymbidium YT57 was insert-transplantedone by one into each cell. The insertion could be performed very easily,and the root could fit well with the support without being damaged. Theplants were cultivated for 45 days in a culture chamber (25° C., 3500Lux, 16h-daytime), and the leaf length, root length, fresh weight, andnumber of roots of each plant were measured. During the cultivation,ion-exchange water was supplied with a syringe until the entire volumeof the cell was filled therewith.

As a result, the plants were well grown when the hydrogel-formingpolymers of Examples 1, 2, and 3 were used. The roots of plants weredecayed during the cultivation when Acryhope, Diawet, and Sumicagel wereused. It is presumed that the plant suffered a calcium deficiency statewhen any of Acryhope, Diawet, and Sumicagel was used.

As a control group, cultivation experiments were conducted in the samemanner as that described above except that each of agar (10 g/L), bark(sold by Mukoyama Orchid Ltd.; bark produced in New Zealand; trade name:MO-2), and sphagnum was used as a support instead of the supportcomprising the hydrogel-forming polymer and pearlite.

As a result, in the case of the agar, insert-transplantation was easybut the root was decayed during the cultivation. In the case of the barkand sphagnum, the insert-transplantation was impossible, and each ofthese support was disposed around the root of the plant and then wastransplanted into the above cells, but such an operation somewhatdamaged the root. Further, in this case, the root was decayed in thecourse of the cultivation. It is presumed that such a phenomenon isattributable to the fact that the agar, bark, and sphagnum have weakwater-absorbing force, and the surrounding of the root was filled withwater, whereby the root suffers deficiency in oxygen.

The thus obtained results are summarized in the following Table 7.

TABLE 7 Growth Evaluation Test of Dtps. (Happy Valentine × Show Girl)‘Mai’ Average Fresh leaf Average root weight Average root length length(g/one number (number Support (cm) (cm) plant) of roots) Example 1 +Pearlite 3.37 4.67 0.78 2.7 Example 2 + Pearlite 2.50 3.75 0.58 3.0Example 3 + Pearlite 2.97 3.07 0.61 3.0 Example 4 + Pearlite 3.07 4.000.70 3.0 Acryhope + Pearlite Measurement was impossible since the rootdied Diawet + Pearlite Sumicagel + Pearlite Agar Bark Peat-moss

Example 14 Cultivating Method Using Thermo-sensitive Water-RetainingSupport

Into 95 ml of the synthetic water shown in Table 2, 95 mg of Hyponexpowder (Hyponex 20-20-20; mfd. by Hyponex Japan K. K.) was dissolved.Into the resultant solution, 1 g of the water-retaining support powderprepared in Example 5 and 100 cc of pearlite was added and uniformlymixed. With the thus obtained support, each cell of the cell tray usedin Example 13 was filled. One plantlet (fresh weight: 1.39 g) of onegenus of orchid plant family, phalaenopsis (Phal. Musashino ‘MH’×Phal.White Moon ‘M-23’), was insert-transplanted in each cell. The insertioncould be performed quite easily, and the root could fit well with thesupport without being damaged. After the plantlets were cultivated for70 days in a greenhouse, the leaf length, root length, and the totalfresh weight of each plant was measured. Watering during the cultivationwas conducted almost everyday from the upper face automatic watering, or30 minutes of capillary watering.

As a control group, a combination of bark (MO-2): sphagnum (Elein PoloCo., Ltd.; produced in Finland):pearlite=6:3:1 (volume ratio) was used.Since the insert-transplantation using this support was impossible, theabove support was disposed around the root of the plant and then wastransplanted into cells, which somewhat damaged the root at thetransplanting.

As shown in the following Table 8, in each watering method, bettergrowth of plant was observed in the cultivation using thewater-retaining support according to the present invention as thesupport, as compared with that in the case of the cultivation using theconventional supports.

TABLE 8 Growth Evaluation Test of Phal. Musashino ‘MH’ × Phal. WhiteMoon ‘M-23’ Root weight Fresh Support Shoot weight (g) (g) weight (g)Watering method: upper face automatic watering Example 5 2.09 1.31 3.40Bark + peat + pearlite 1.82 1.11 2.93 Watering method: capillarywatering Example 5 2.43 2.05 4.48 Bark + peat + pearlite 2.24 1.40 3.64

Example 15

(Cultivating Method Using Water-Retaining support

Into 1 g of the dried polymer powder prepared in Example 1, 100 ml of aHyponex solution (Hyponex 20-20-20, 1 g/L, dissolved in synthetic water)was completely absorbed so as to form a gel. The pF value of the thusobtained gel was measured by a pF meter (manufacture by Daiki Rika KogyoK. K., DIK-8340), and the value was found to be 0 (zero). The gel wastransferred to a 9-cm diameter black plastic pot (available from SaegusaShigeo Shoten; diameter: 7.5 cm), and the total weight was measured.With no watering at all, the plastic pot was left standing in agreenhouse, and the total weight and pF value thereof was measured threetimes at 24, 48, and 72 hours thereafter.

In this measurement, the following formulas were used.

Water content at each point=weight at each point−1 g (weight of driedpolymer)−weight of black vinyl pot

Initial value (value at starting) to be 1 (one),

Nutrient concentration of the solution at each point was determined as:

nutrient concentration at each point=initial water content/water contentat each point.

Comparative Example 10 Cultivating Method Using Bark

The weight and moisture content of 100 ml of bark was measured and itwas found to be 30.93 g and 35.7%, respectively. 100 ml of undried barkwas soaked in the Hyponex solution used in Example 15 for 24 hours. Thethus moisturized bark was scooped up with a net, and the surplus waterwas removed. The weight and pF value of the water-retaining bark was46.56 g and 0, respectively. After the bark was transferred to the blackplastic pot, the total weight thereof was measured, the bark was leftstanding in a greenhouse, and the total weight and pF value thereof wasmeasured three times at 24, 48, and 72 hours after the initialmeasurement. The water content and concentration at each point wasdetermined by using the formulas in the same manner as those in Example15.

initial water content=30.93 (weight of 100 ml of bark)×0.357 (moisturecontent)+46.56 (water-retaining bark weight)−30.93 (weight of 100 ml ofbark)=26.67

water content at each point=initial water content−(initial-total weightof vessel−weight at each point)

initial concentration of Example 15 to be 1,

initial concentration of the solution was determined as:

initial concentration of solution=26.67÷(30.93×0.357+26.67)=0.71

nutrient concentration of solution at each point=initial watercontent/water content at each point

When the nutrient content in Example 15 was considered to be 1, thenutrient content of Comparative Example=26.6×0.71÷100=0.19.

As shown in the following Table 9, when the water-retaining supportaccording to the present invention is used as a cultivating support,since its moisture ratio is high, as compared with that in the case ofthe bark, the nutrient content may be made greater in the vessel havingthe same volume, and the fluctuation in the nutrient concentrationduring the culture may be made.smaller. Further, since a large amount ofwater may be retained for a long period, the frequency of watering maybe reduced, and the risk of plant being exposed to water stress may beavoided.

TABLE 9 Changes in pF, Water Content, Nutrient Concentration, andNutrient Content with Elapse of Time Example 1 + water Bark + waterElapsed Water Nutrient Water Nutrient time content concent- Nutrientcontent concent- Nutrient (hs) pF (cc) ration content pF (cc) rationcontent Initial 0 100  1.00 1.00 0 27 0.71 0.19 24 0 78 1.28 1.00 0 171.11 0.19 48 0 67 1.50 1.00 0.5 10 1.90 0.19 72 0 56 1.80 1.00 2.0  53.80 0.19

As shown in the above Table 9, the following results were obtained.

The initial water content retained in Comparative Example was 27% on thebasis of that of Example.

The initial nutrient content retained in Comparative Example was 19% onthe basis of that of Example.

The nutrient concentration at the elapsed time of 72 hours was 1.8 timesthe initial concentration in Example, and the nutrient concentration atthe elapsed time of 72 hours was 3.8 times the initial concentration inComparative Example.

The residual water content at the elapsed time of 72 hours was 56 cc inExample and 5 cc in Comparative Example.

The pF value at the elapsed time of 72 hours was 0 in Example and 2.0 inComparative Example.

Industrial Applicability

As described hereinabove, according to the present invention, there isprovided a water-retaining support for plant comprising ahydrogel-forming polymer having a calcium ion absorption of less than 50mg per 1 g of the dry weight thereof and having a water absorptionmagnification in ion-exchange water (at room temperature; 25° C.) of 100or more.

The present invention also provides a water-retaining support for plantcomprising a hydrogel-forming polymer having a carboxyl group bonded tothe polymer chain thereof, and having a content of alkali metal salt orammonium salt of the carboxyl group of 0.3 to 2.5 mmol per 1 g of thedry weight of the support.

When the water-retaining support for plant according to the presentinvention is used, since the water-retaining support absorbs thereinonly a small amount of calcium ion, a plant does not suffer from calciumion deficiency. In addition, since the water absorption magnification ofsuch a support is sufficiently large, the support can supply sufficientwater to a plant.

The present invention further provides a plant-growing vessel comprisinga vessel-shaped substrate capable of accommodating therein at least aportion of a plant; and a water-retaining support for plant disposed inthe vessel-shaped substrate and having a crosslinked structure.

The present invention further provides a plant-growing sheet comprisinga sheet-shaped substrate; and a water-retaining support for plantdisposed on at least one surface of the substrate and having acrosslinked structure.

When the plant-growing vessel or sheet according to the presentinvention is used, on the basis of the characteristic (capacity to storewater or nutrient, or the temperature dependency thereof) of thehydrogel-forming polymer which is disposed on the plant side of thevessel or sheet, and has a crosslinked structure, the volume ofplant-growing vessel may be reduced markedly, thereby to improve theroot origination ratio, to reduce the area required for plant growth, toreduce the amount of material required for a plant-growing vessel, andto decrease the transporting cost. Further, the cost may greatly bereduced by the labor-saving in water control, etc.

What is claimed is:
 1. A water-retaining support for a plant comprisinga hydrogel-forming polymer having a calcium ion absorption of less than50 mg per g of dry weight thereof and having a water absorptionmagnification in ion-exchange water at room temperature, represented bythe formula (W₂−W₁)/W₁, of 100 or more, wherein W₁ is the weight of thedry hydrogel-forming polymer and W₂ is the weight of thehydrogel-forming polymer after immersion in excess of ion-exchange waterat room temperature for 48 hours.
 2. The water-retaining support for aplant according to claim 1, wherein the hydrogel-forming polymer is apolymer having a carboxyl group bonded to the polymer chain thereof, andthe content of alkali metal salt or ammonium salt of the carboxyl groupis 0.3 to 2.5 mmol per g of dry weight of the support.
 3. Thewater-retaining support for a plant according to claim 1, wherein thehydrogel-forming polymer is a polymer such that the total amount oforganic material remaining in a liquid which has been obtained byextracting the polymer with distilled water in an amount 1,000 timesthat of the polymer is 15 ppm or less, in terms of the value of chemicaloxygen demand.
 4. The water-retaining support for a plant according toclaim 1, wherein the hydrogel-forming polymer is a polymer such that thetotal amount of volatile carboxylic acid and salt thereof remaining in 1g of dry weight of the polymer is 0.5 mmol or less.
 5. Thewater-retaining support for a plant according to claim 1, wherein thehydrogel-forming polymer is a polymer containing at least 3 mmol of acarboxyl group bonded to the polymer chain thereof per g of dry weight,and the content of alkali metal salt or ammonium salt of the carboxylgroup is 0.3 to 2.5 mmol per g of dry weight of the support.
 6. Thewater-retaining support for a plant according to claim 5, wherein thehydrogel-forming polymer is a polyacrylic acid-type polymer.
 7. Thewater-retaining support for a plant according to claim 5, which furthercontains a calcium salt of a carboxyl group.
 8. The water-retainingsupport for a plant according to claim 1, wherein the crosslinking ratioin the neighborhood of the surface of the hydrogel-forming polymer ishigher than the crosslinking ratio in the inside thereof.
 9. Thewater-retaining support for a plant according to claim 1, wherein thehydrogel-forming polymer is a polymer showing a decrease in waterabsorption magnification along with an increase in temperature within atemperature range of not lower than 0° C. and not higher than 70° C. andshowing a water absorption magnification which is reversibly changeablewith respect to temperature.
 10. The water-retaining support for a plantaccording to claim 1, further including a porous material.
 11. Awater-retaining support for a plant comprising a weakly acidichydrogel-forming polymer.
 12. The water-retaining support for a plantaccording to claim 11, wherein the weakly acidic hydrogel-formingpolymer has a calcium ion absorption of less than 50 mg per g of dryweight and a water absorption magnification in ion-exchange water atroom temperature, represented by the formula (W₂−W₁)W₁, of 100 or more,wherein W₁ is the weight of the dry hydrogel-forming polymer and W₂ isthe weight of the hydrogel-forming polymer after immersion in excess ofion-exchange water at room temperature for 48 hours.
 13. Thewater-retaining support for a plant according to claim 11, wherein thehydrogel-forming polymer is a polymer showing a decrease in waterabsorption magnification, represented by the formula (W₂−W₁)/W₁, whereinW₁ is the weight of the dry hydrogel-forming polymer and W₂ is theweight of the hydrogel-forming polymer after immersion in excess ofion-exchange water at room temperature for 48 hours, along with anincrease in temperature within a temperature range of not lower than 0°C. and not higher than 70° C. and showing a water absorptionmagnification which is reversibly changeable with respect totemperature.
 14. A plant-growing support comprising a water-retainingsupport for a plant comprising a hydrogel-forming polymer having acalcium ion absorption of less than 50 mg per g of dry weight thereofand having a water absorption magnification in ion-exchange water atroom temperature, represented by the formula (W₂−W₁)/W₁, of 100 or more,wherein W₁ is the weight of the dry hydrogel-forming polymer and W₂ isthe weight of the hydrogel-forming polymer after immersion in excess ofion-exchange water at room temperature for 48 hours, and a nutrientand/or plant growth-regulating substance retained in the support.
 15. Aplant-growing vessel comprising a vessel-shaped substrate capable ofaccommodating at least a portion of a plant and a water-retainingsupport for a plant comprising a hydrogel-forming polymer having acalcium ion absorption of less than 50 mg per g of dry weight thereofand having a water absorption magnification in ion-exchange water atroom temperature, represented by the formula (W₂−W₁)/W₁, of 100 or more,wherein W₁ is the weight of the dry hydrogel-forming polymer and W₂ isthe weight of the hydrogel-forming polymer after immersion in excess ofion-exchange water at room temperature for 48 hours, disposed in thevessel-shaped substrate.
 16. The plant-growing vessel according to claim15, wherein the water-retaining support for a plant is retained insideof the vessel in a fixed state.
 17. A plant-growing support comprising awater-retaining support for a plant comprising a weakly acidichydrogel-forming polymer and a nutrient and/or plant growth-regulatingsubstance retained in the support.
 18. A plant-growing supportcomprising a water-retaining support for a plant comprising ahydrogel-forming polymer having a calcium ion absorption of less than 50mg per g of dry weight thereof and having a water absorptionmagnification in ion-exchange water at room temperature, represented bythe formula (W₂−W₁)/W₁, of 100 or more, wherein W₁ is the weight of thedry hydrogel-forming polymer and W₂ is the weight of thehydrogel-forming polymer after immersion in excess of ion-exchange waterat room temperature for 48 hours, wherein the hydrogel-forming polymeris a polymer showing a decrease in water absorption magnification alongwith an increase in temperature within a temperature range of not lowerthan 0° C. and not higher than 70° C. and showing a water absorptionmagnification which is reversibly changeable with respect to temperatureand a nutrient and/or plant growth-regulating substance retained in thesupport.
 19. A plant-growing support comprising a water-retainingsupport for a plant comprising a hydrogel-forming polymer having acalcium ion absorption of less than 50 mg per g of dry weight thereofand having a water absorption magnification in ion-exchange water atroom temperature, represented by the formula (W₂−W₁)/W₁, of 100 or more,wherein W₁ is the weight of the dry hydrogel-forming polymer and W₂ isthe weight of the hydrogel-forming polymer after immersion in excess ofion-exchange water at room temperature for 48 hours, and a porousmaterial and a nutrient and/or plant-growth-regulating substanceretained in the support.
 20. A plant-growing vessel comprising avessel-shaped substrate capable of accommodating at least a portion of aplant and a water-retaining support for a plant comprising a weaklyacidic hydrogel-forming polymer disposed in the vessel-shaped substrate.21. The plant-growing vessel according to claim 20, wherein thewater-retaining support for a plant is retained inside of the vessel ina fixed state.
 22. A plant-growing vessel comprising a vessel-shapedsubstrate capable of accommodating at least a portion of a plant and awater-retaining support for a plant comprising a hydrogel-formingpolymer having a calcium ion absorption of less than 50 mg per g of dryweight thereof and having a water absorption magnification inion-exchange water at room temperature, represented by the formula(W₂−W₁)/W₁, of 100 or more, wherein W₁ is the weight of the dryhydrogel-forming polymer and W₂ is the weight of the hydrogel-formingpolymer after immersion in excess of ion-exchange water at roomtemperature for 48 hours, wherein the hydrogel-forming polymer is apolymer showing a decrease in water absorption magnification along withan increase in temperature within a temperature range of not lower than0° C. and not higher than 70° C. and showing a water absorptionmagnification which is reversibly changeable with respect to temperaturedisposed in the vessel-shaped substrate.
 23. The plant-growing vesselaccording to claim 22, wherein the water-retaining support for a plantis retained inside of the vessel in a fixed state.
 24. A plant-growingvessel comprising a vessel-shaped substrate capable of accommodating atleast a portion of a plant and a water-retaining support for a plantcomprising a hydrogel-forming polymer having a calcium ion absorption ofless than 50 mg per g of dry weight thereof and having a waterabsorption magnification in ion-exchange water at room temperature of100 or more and a porous material disposed in the vessel-shapedsubstrate.
 25. The plant-growing vessel according to claim 24, whereinthe water-retaining support for a plant is retained inside of the vesselin a fixed state.